I. Very short answer type question:
Ans. The aqueous humour nourishing the cornea and the lens by supplying nutrition such as amino acids and glucose, the aqueous and maintain intraocular pressure.
2. When we enter from bright sunlight to a dark room, we can not see initially. Why?
Ans. The pupil of an eye acts like a variable aperture whose size can be varied with the help of the iris and the adjustment of the pupil takes time. So, when we enter from bright sunlight to a dark room, we cannot see initially.
3. A person uses spectacles of power +2D. What is the defect of vision he is suffering from?
Ans. A person who uses spectacles of power +2D means he is suffering from hypermetropia (long-sightedness).
4. Why do chickens wake up early and sleep early?
Ans. Chickens have a large number of rod cells that help them to detect the intensity of light. Thus, chickens wake up early and go to sleep early.
5. What is the nature of the image formed at retina?
Ans. The image formed at the retina is diminished, inverted and real.
6. What is the cause of colour blindness?
Ans. Cone cells of the retina are sensitive to colours and when these cells do not respond properly, enable the retina to distinguish between colors.
7. State the structure of iris and its functions in the human eye.
Ans. A structure called iris behind the cornea is a dark muscular diaphragm that controls the size of the pupil and the pupil regulates and controls the amount of light.
8. Define the distance of distinct vision and give its range.
Ans. The minimum distance, at which objects can be seen most distinctly without strain, is called the least distance of distinct vision and its range is about 25 cm.
9. What is meant by the least distance of distinct vision?
Ans. The least distance of distinct vision means the minimum distance, at which objects can be seen most distinctly without strain.
10. Define the power of accommodation of the eye.
Ans. The ability of the eye lens to adjust its focal length is called the power of accommodation.
11. Why the clear sky appear blue?
Ans. When sunlight passes through the atmosphere the fine particles in the air scatter the blue colour, so the clear sky appears blue.
12. Why does it take some time to see objects in a cinema hall when we just entered the hall from bright sunlight? Explain in brief.
Ans. The pupil of an eye acts like a variable aperture whose size can be varied with the help of the iris and the adjustment of the pupil takes time. So, it takes some time to see objects in a cinema hall when we just entered the hall from bright sunlight.
13. How does the thickness of the eye lens change when we shift looking from a distance tree to reading a book?
Ans. The thickness of the eye lens increases when we shift looking from a distance tree to reading a book.
14. A student sitting at the back of the classroom cannot read clearly the letters written on the blackboard. What advice will a doctor give to her?
Ans. The student is a short-sightedness or Myopia and a doctor will give her advice to take a spectacle of -ve power means the concave lens of suitable power.
.
15. A hyper meteoric person prefers to remove his spectacles while driving. Give reason.
Ans. A person with hypermetropia can see distant objects clearly and during driving a person has to see more than a nearer point (25 cm). This is because
a hyper meteoric person prefers to remove his spectacles while driving.
16. How are we able to see nearby and also the distant objects clearly?
Ans. We are able to see nearby and also the distant objects clearly by the ability of the eye lens to adjust its focal length that is called power accommodation.
17. Why do parallel rays of different colours deviate differently while passing through a glass prism?
Ans. Different colours of light bend through different angles with respect to the incident ray while passing through a prism as they have different wavelengths.
18. Name any two phenomena associated with the formation of the rainbow.
Ans. Two phenomena associated with the formation of the rainbow are internal reflection and dispersion.
19. Draw a ray diagram showing the dispersion through a Prism when a narrow beam of white light is incident on one of its refracting surfaces. Also, indicate the order of the colours of the spectrum obtained.
Ans. Ans.
20. Define the angle of deviation.
Ans. The angle between the incident ray and emergent ray is called the angle of deviation.
21. List the colours into which light splits in the decreasing order of their bending on emergence from the prism.
Ans. Red, orange, yellow, green, blue, indigo and violet.
22. A beam of white light splits when it passes through a Prism. Name this phenomenon and give its reason.
Ans. The phenomenon is refraction and the reason is the different wavelengths of a different colour and different colour deviate from different angles.
23. Why does the sun look reddish at the time of sunrise and sunset? Explain.
Ans. During sunrise and sunset, light from the Sun near the horizon passes through thicker layers of air and larger distance in the earth’s atmosphere. Shorter wavelengths are scattered away by the particles and most of the red light of a longer wavelength which is least scattered reaches our eyes. This gives rise to the reddish appearance of the Sun.
24. Why do different components of white light split up into a spectrum, when it passes through a triangular glass prism?
Ans. Different colours of light bend through different angles with respect to the incident ray while passing through a prism as different colours have different wavelengths so deviate from different angles.
25. What is the dispersion?
Ans. The splitting of light into its seven component colours is called dispersion.
26. What happens when light is passed through a glass prism.
Ans. Different colours of light bend through different angles with respect to the incident ray, as they pass through a prism.
27. What is astigmatism?
Ans. Astigmatism is a common vision problem caused by irregular-shaped of cornea, that causes blurred vision.
28. Name the defect of vision in which the eye loses its power of accommodation due to old age.
Ans. Presbyopia.
II. Short answer type questions:
3. A person uses spectacles of power +2D. What is the defect of vision he is suffering from?
Ans. A person who uses spectacles of power +2D means he is suffering from hypermetropia (long-sightedness).
4. Why do chickens wake up early and sleep early?
Ans. Chickens have a large number of rod cells that help them to detect the intensity of light. Thus, chickens wake up early and go to sleep early.
5. What is the nature of the image formed at retina?
Ans. The image formed at the retina is diminished, inverted and real.
6. What is the cause of colour blindness?
Ans. Cone cells of the retina are sensitive to colours and when these cells do not respond properly, enable the retina to distinguish between colors.
7. State the structure of iris and its functions in the human eye.
Ans. A structure called iris behind the cornea is a dark muscular diaphragm that controls the size of the pupil and the pupil regulates and controls the amount of light.
8. Define the distance of distinct vision and give its range.
Ans. The minimum distance, at which objects can be seen most distinctly without strain, is called the least distance of distinct vision and its range is about 25 cm.
9. What is meant by the least distance of distinct vision?
Ans. The least distance of distinct vision means the minimum distance, at which objects can be seen most distinctly without strain.
10. Define the power of accommodation of the eye.
Ans. The ability of the eye lens to adjust its focal length is called the power of accommodation.
11. Why the clear sky appear blue?
Ans. When sunlight passes through the atmosphere the fine particles in the air scatter the blue colour, so the clear sky appears blue.
12. Why does it take some time to see objects in a cinema hall when we just entered the hall from bright sunlight? Explain in brief.
Ans. The pupil of an eye acts like a variable aperture whose size can be varied with the help of the iris and the adjustment of the pupil takes time. So, it takes some time to see objects in a cinema hall when we just entered the hall from bright sunlight.
13. How does the thickness of the eye lens change when we shift looking from a distance tree to reading a book?
Ans. The thickness of the eye lens increases when we shift looking from a distance tree to reading a book.
14. A student sitting at the back of the classroom cannot read clearly the letters written on the blackboard. What advice will a doctor give to her?
Ans. The student is a short-sightedness or Myopia and a doctor will give her advice to take a spectacle of -ve power means the concave lens of suitable power.
.
15. A hyper meteoric person prefers to remove his spectacles while driving. Give reason.
Ans. A person with hypermetropia can see distant objects clearly and during driving a person has to see more than a nearer point (25 cm). This is because
a hyper meteoric person prefers to remove his spectacles while driving.
16. How are we able to see nearby and also the distant objects clearly?
Ans. We are able to see nearby and also the distant objects clearly by the ability of the eye lens to adjust its focal length that is called power accommodation.
17. Why do parallel rays of different colours deviate differently while passing through a glass prism?
Ans. Different colours of light bend through different angles with respect to the incident ray while passing through a prism as they have different wavelengths.
18. Name any two phenomena associated with the formation of the rainbow.
Ans. Two phenomena associated with the formation of the rainbow are internal reflection and dispersion.
19. Draw a ray diagram showing the dispersion through a Prism when a narrow beam of white light is incident on one of its refracting surfaces. Also, indicate the order of the colours of the spectrum obtained.
Ans. Ans.
20. Define the angle of deviation.
Ans. The angle between the incident ray and emergent ray is called the angle of deviation.
21. List the colours into which light splits in the decreasing order of their bending on emergence from the prism.
Ans. Red, orange, yellow, green, blue, indigo and violet.
22. A beam of white light splits when it passes through a Prism. Name this phenomenon and give its reason.
Ans. The phenomenon is refraction and the reason is the different wavelengths of a different colour and different colour deviate from different angles.
23. Why does the sun look reddish at the time of sunrise and sunset? Explain.
Ans. During sunrise and sunset, light from the Sun near the horizon passes through thicker layers of air and larger distance in the earth’s atmosphere. Shorter wavelengths are scattered away by the particles and most of the red light of a longer wavelength which is least scattered reaches our eyes. This gives rise to the reddish appearance of the Sun.
24. Why do different components of white light split up into a spectrum, when it passes through a triangular glass prism?
Ans. Different colours of light bend through different angles with respect to the incident ray while passing through a prism as different colours have different wavelengths so deviate from different angles.
25. What is the dispersion?
Ans. The splitting of light into its seven component colours is called dispersion.
26. What happens when light is passed through a glass prism.
Ans. Different colours of light bend through different angles with respect to the incident ray, as they pass through a prism.
27. What is astigmatism?
Ans. Astigmatism is a common vision problem caused by irregular-shaped of cornea, that causes blurred vision.
28. Name the defect of vision in which the eye loses its power of accommodation due to old age.
Ans. Presbyopia.
II. Short answer type questions:
1. What is meant by presbyopia? Mention two causes due to which presbyopia occurs. Explain how it is corrected.
Ans. The power of accommodation of the eye usually decreases with ageing. For most people, the near point gradually recedes away. They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect is called Presbyopia.
It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.
It is corrected by a common type of bi-focal lenses consists of both concave and convex lenses.
2. A person can see clearly up to 3 m. Describe the type of lens that should be used so that he can see up to 12 m clearly.
Ans. The person is shortsightedness. He can see nearby objects clearly but cannot see distant objects distinctly. In a myopic eye, the image of a distant object is formed in front of the retina. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.
3. A far point of a nearsighted person is 1.5m. Find the nature and power of the lens required.
Ans. A far point of a nearsighted person is 1.5m, he can see nearby objects clearly but cannot see distant objects distinctly. In a myopic eye, the image of a distant object is formed in front of the retina. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.
7. (a) Make a ray diagram to show how the eye defect myopia is corrected by using a suitable lens.
11. Study the diagram given below and answer the following question.
12. (a) Define the power of a lens and write its SI unit.
23. Name the phenomenon associated with the following:
Ans. The power of accommodation of the eye usually decreases with ageing. For most people, the near point gradually recedes away. They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect is called Presbyopia.
It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.
It is corrected by a common type of bi-focal lenses consists of both concave and convex lenses.
2. A person can see clearly up to 3 m. Describe the type of lens that should be used so that he can see up to 12 m clearly.
Ans. The person is shortsightedness. He can see nearby objects clearly but cannot see distant objects distinctly. In a myopic eye, the image of a distant object is formed in front of the retina. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.
3. A far point of a nearsighted person is 1.5m. Find the nature and power of the lens required.
Ans. A far point of a nearsighted person is 1.5m, he can see nearby objects clearly but cannot see distant objects distinctly. In a myopic eye, the image of a distant object is formed in front of the retina. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.
4. What power of the lens will change the reading distance from 1 m to 0. 25 m?
Ans. When we are looking at objects closer to the eye, the focal length of the lens decreases, therefore, power increases as the power of the lens is inversely proportional to the focus. P = 1/f.
Ans. When we are looking at objects closer to the eye, the focal length of the lens decreases, therefore, power increases as the power of the lens is inversely proportional to the focus. P = 1/f.
5. A person is unable to see the object nearest than 50 cm. He wants to read a book placed at a distance of 25 cm. Find the nature, focal length and power of the lens are required for his spectacles.
Ans. The person is hypermetropic or long-sightedness. To read a book placed at a distance of 25 cm. the spectacles should be of a convex lens, less focal length, and suitable +ve power.
Ans. The person is hypermetropic or long-sightedness. To read a book placed at a distance of 25 cm. the spectacles should be of a convex lens, less focal length, and suitable +ve power.
6. (a) What is meant by the least distance of distinct vision?
(b) How does iris control the size of the pupil in bright light and dim light?
Ans. (a) The minimum distance, at which objects can be seen most distinctly without strain, is called the least distance of distinct vision or near the point of the eye. For a young adult with normal vision, the near point is about 25 cm.
(b) Iris controls the size of the pupil and the pupil regulates and controls the amount of light entering the eye.
Ans. (a) The minimum distance, at which objects can be seen most distinctly without strain, is called the least distance of distinct vision or near the point of the eye. For a young adult with normal vision, the near point is about 25 cm.
(b) Iris controls the size of the pupil and the pupil regulates and controls the amount of light entering the eye.
7. (a) Make a ray diagram to show how the eye defect myopia is corrected by using a suitable lens.
(b) State two reasons due to which the myopia eye defect may be caused?
Ans. (a)
(b) This defect may arise due to
(i) excessive curvature of the eye lens, or
(ii) elongation of the eyeball.
Ans. (a)
(b) This defect may arise due to
(i) excessive curvature of the eye lens, or
(ii) elongation of the eyeball.
8. Is the position of a star as seen by us its true position? Justify your answer.
Ans. No, the position of a star as seen by us is not it's true position. The atmospheric refraction occurs in a medium of gradually changing the refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. The star appears slightly higher than its actual position when viewed near the horizon.
9. Why do we see a rainbow in the sky only after a rainfall?
Ans. A rainbow is a natural spectrum appearing in the sky after a rain shower because it is caused by the dispersion of sunlight by tiny water droplets, present in the atmosphere. The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop.
Ans. A rainbow is a natural spectrum appearing in the sky after a rain shower because it is caused by the dispersion of sunlight by tiny water droplets, present in the atmosphere. The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop.
10. (a) Draw a diagram to show the formation of the image of a distant object by a myopic eye. How can such an eye defect be rectified?
(b) State two reasons due to which this eye defect may be caused.
Ans. See the answer of Q. 7
Ans. See the answer of Q. 7
11. Study the diagram given below and answer the following question.
(i) Name the defect of vision defeated in the diagram.
(ii) List two causes of the defects.
(iii) Draw a ray diagram for the correction of the above defects using an appropriate lens.
Ans. (i) Hypermetropia or far-sightedness.
(ii) Two causes of the defects are (a) the focal length of the eye lens is too long, or (b) the eyeball has become too small.
(iii)
Ans. (i) Hypermetropia or far-sightedness.
(ii) Two causes of the defects are (a) the focal length of the eye lens is too long, or (b) the eyeball has become too small.
(iii)
12. (a) Define the power of a lens and write its SI unit.
(b) A convex lens of power 4D is placed at a distance of 40 cm from a wall. At what distance from the lens should be a candle placed so that its image is formed on the wall?
Ans. (a) The power of the lens is the ability of the lens to converge or diverge the rays of light falling on it. It is defined as P=1/f where f is the focal length of the lens.
S.I. unit of power is Dioptre D. It is represented by 'P'.
(b) Given Power = 4D
P = 1/f
4 D = 1/f
f = 1/4 m = 1/4 x 100cm = +25 cm.
Image distance = +40 cm.
Object distance =?
We have 1/f=1/v - 1/u
1/u = 1/v -1/f = 1/40 - 1/25 =(25 - 40) /1000
1 /u = 15/1000=- 3/200
u = - 200/3= -66.67 cm.
The distance between the candle and lens should be 66.67cm.
Ans. (a) The power of the lens is the ability of the lens to converge or diverge the rays of light falling on it. It is defined as P=1/f where f is the focal length of the lens.
S.I. unit of power is Dioptre D. It is represented by 'P'.
(b) Given Power = 4D
P = 1/f
4 D = 1/f
f = 1/4 m = 1/4 x 100cm = +25 cm.
Image distance = +40 cm.
Object distance =?
We have 1/f=1/v - 1/u
1/u = 1/v -1/f = 1/40 - 1/25 =(25 - 40) /1000
1 /u = 15/1000=- 3/200
u = - 200/3= -66.67 cm.
The distance between the candle and lens should be 66.67cm.
13. Ravi kept a book at a distance of 10 cm from the eyes of his friend Hari. Hari is not able to read anything written on the books. Explain why?
Ans. A normal eye can see objects clearly that are between 25 cm and infinity.
Ravi kept a book at a distance of 10 cm that is less than 25 cm from the eyes of his friend Hari so Hari is not able to read anything written on the books and the image is blurred or feels strain in the eye.
Ans. A normal eye can see objects clearly that are between 25 cm and infinity.
Ravi kept a book at a distance of 10 cm that is less than 25 cm from the eyes of his friend Hari so Hari is not able to read anything written on the books and the image is blurred or feels strain in the eye.
14. A lens of the focal length of 5.0 cm is being used by a student in the laboratory as a magnifying glass. His least distance of distinct vision is 25 cm. What magnification is the student getting?
Ans. Given: focal length f = 5.0 cm
Object distance = -25 cm.
V=?
We have a lens formula: 1/f = 1/v - 1/u
1/v = 1/f +1/u = 1/5 + 1/-25 = 1/5 - 1/25
1/v = 4/25 cm
v = 25/4 cm = 6.25 cm.
Magnification m = v/u = 6.25/-25 = - 625/2500
m = - 5/100 = -1/4= - 0. 25 .
Ans. Given: focal length f = 5.0 cm
Object distance = -25 cm.
V=?
We have a lens formula: 1/f = 1/v - 1/u
1/v = 1/f +1/u = 1/5 + 1/-25 = 1/5 - 1/25
1/v = 4/25 cm
v = 25/4 cm = 6.25 cm.
Magnification m = v/u = 6.25/-25 = - 625/2500
m = - 5/100 = -1/4= - 0. 25 .
15. The near point of a person suffering from hypermetropia is 75 cm. Calculate the focal length and power of the lens required to enable him to read the newspaper which is kept at 25 cm from the eye.
Ans. Given :
u = -25 cm
v = -75 cm
From lens formula : 1/f = 1/v - 1/u
1/f = -1/25 - (-1/75) = -1/75 +1/25 = 2/75
1/f = 2/75
f = 75/2 = 37.5cm = 0.375 m = 0.38m
Power P of the lens = 1/f = 1/0.38 = 100/38 = +2.63Dcm
Focal length = 37.5 m and power of the lens = +2.63D.
Ans. Given :
u = -25 cm
v = -75 cm
From lens formula : 1/f = 1/v - 1/u
1/f = -1/25 - (-1/75) = -1/75 +1/25 = 2/75
1/f = 2/75
f = 75/2 = 37.5cm = 0.375 m = 0.38m
Power P of the lens = 1/f = 1/0.38 = 100/38 = +2.63Dcm
Focal length = 37.5 m and power of the lens = +2.63D.
16. What will be the colour of the sky be for an astronaut staying in the International Space Station orbiting the earth? Justify your answer by giving reasons.
Ans. The colour of the sky will be black for an astronaut staying in the International Space Station orbiting the earth because there is no atmosphere in the space and the light reaching it does not scatter. Scattering of blue light of short wavelength causes the blue colour of the sky.
Ans. The colour of the sky will be black for an astronaut staying in the International Space Station orbiting the earth because there is no atmosphere in the space and the light reaching it does not scatter. Scattering of blue light of short wavelength causes the blue colour of the sky.
17. Why does the sun seem to rise two minutes before the actual sunrise and sets two minutes after the actual sunset? Explain with the help of the labelled diagram.
Ans. The sun seems to rise two minutes before the actual sunrise and sets two minutes after the actual sunset because of atmospheric refraction. By actual sunrise, we mean the actual crossing of the horizon by the Sun shows the actual and apparent positions of the Sun with respect to the horizon. The time difference between actual sunset and the apparent sunset is about 2 minutes.
Ans. The sun seems to rise two minutes before the actual sunrise and sets two minutes after the actual sunset because of atmospheric refraction. By actual sunrise, we mean the actual crossing of the horizon by the Sun shows the actual and apparent positions of the Sun with respect to the horizon. The time difference between actual sunset and the apparent sunset is about 2 minutes.
18. Define the angle of deviation. Why do different components of white light split up into spectrum when it passes through a triangular glass prism. Show the angle of the deviation for red colour when white light passes through a prism.
Ans. The angle between the incident ray and emergent ray is called the angle of deviation.
Different components of white light split up into spectrum when it passes through a triangular glass prism because different colour has a different wavelength and deviate with different angles.
Different components of white light split up into spectrum when it passes through a triangular glass prism because different colour has a different wavelength and deviate with different angles.
19. States the difference in colour of the sun observed during sunrise, sunset and noon. Give an explanation for each.
Ans. Light from the Sun overhead would travel relatively shorter distances so at noon, the Sun appears white as only a little of the blue and violet colours are scattered. Near the horizon, most of the blue light and shorter wavelengths are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelengths. This gives rise to the reddish appearance of the Sun because the red colour has a longer wavelength.
Ans. Light from the Sun overhead would travel relatively shorter distances so at noon, the Sun appears white as only a little of the blue and violet colours are scattered. Near the horizon, most of the blue light and shorter wavelengths are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelengths. This gives rise to the reddish appearance of the Sun because the red colour has a longer wavelength.
20. Enlist at least six phenomena observed in nature subsequent to the scattering of light.
Ans. Six phenomena observed in nature subsequent to the scattering of light are:
i. Tyndal effect- a fine beam of sunlight enters a smoke-filled room through a small hole.
ii. The colour of the clear Sky Blue.
iii. Colour of the Sun at Sunrise and Sunset red.
iv. The visible path of light when entres a dark room through a hole.
v. Sometimes after the rains, two rainbows are seen due to scattering.
vi. The red colour is used for danger.
Ans. Six phenomena observed in nature subsequent to the scattering of light are:
i. Tyndal effect- a fine beam of sunlight enters a smoke-filled room through a small hole.
ii. The colour of the clear Sky Blue.
iii. Colour of the Sun at Sunrise and Sunset red.
iv. The visible path of light when entres a dark room through a hole.
v. Sometimes after the rains, two rainbows are seen due to scattering.
vi. The red colour is used for danger.
21. What is the Tyndall effect? Give at least two illustrating the Tyndall effect.
Ans. When a beam of light strikes fine particles, the path of the beam becomes visible. The light reaches us, after being reflected diffusely by these particles. The phenomenon of scattering of light by the colloidal particles is called the Tyndall effect.
Two examples: I.When a fine beam of sunlight enters a smoke-filled room through a small hole.
II. The Tyndall effect can also be observed when sunlight passes through a canopy of a dense forest.
Ans. When a beam of light strikes fine particles, the path of the beam becomes visible. The light reaches us, after being reflected diffusely by these particles. The phenomenon of scattering of light by the colloidal particles is called the Tyndall effect.
Two examples: I.When a fine beam of sunlight enters a smoke-filled room through a small hole.
II. The Tyndall effect can also be observed when sunlight passes through a canopy of a dense forest.
23. Name the phenomenon associated with the following:
(a) The sky appears blue.
(b) Formation of a rainbow in the sky.
Ans. (a) The sky appears blue due to the scattering of blue light. When sunlight passes through the atmosphere the fine particles in the air scatter the blue colour, so the clear sky appears blue.
(b) Rainbow forms after rain due to total internal reflection and dispersion of light through tiny water droplets that act as a prism.
Ans. (a) The sky appears blue due to the scattering of blue light. When sunlight passes through the atmosphere the fine particles in the air scatter the blue colour, so the clear sky appears blue.
(b) Rainbow forms after rain due to total internal reflection and dispersion of light through tiny water droplets that act as a prism.
24. Why the power of accommodation of an eye decreases with age? Explain.
Ans. The power of accommodation of the eye usually decreases with ageing. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.
25. Draw ray diagram each show:
(i) Myopic eye (ii) Hypermetropic eye.
Ans. (i) Myopic eye-
(ii) Hypermetropic eye-
26. What is meant by the near point and far point of an eye? State their values for the normal human eyes.
Ans. The minimum distance, at which objects can be seen most distinctly without strain, is called the near point of the eye. For a young adult with normal vision, the near point is about 25 cm. The farthest point up to which the eye can see objects clearly is called the far point of the eye. It is infinity for a normal eye.
27. How is normal eye able to see distinctly distances as well as near objects?
Ans. The eye lens's curvature can be modified to some extent by the ciliary muscles. When the muscles are relaxed, the lens becomes thin. Thus, its focal length increases. This enables us to see distant objects clearly.
When we are looking at objects closer to the eye, the ciliary muscles contract. This increases the curvature of the eye lens and the focal length of the eye lens decreases. This enables us to see nearby objects clearly.
28. A person needs a lens of power - 4.5D for correction of her vision.
(i) What kind of defect in visual is she suffering from?
(ii) What is the focal length of the corrective lens?
(iii) What is the nature of the corrective lens?
Ans. (i) She is suffering from myopia.
(b) Given: Power P = - 4.5D
We have P = 1/f
f = 1/Pm.
f= 1/-4.5m
f= - 1000/45 cm= -22.2cm.
(iii) Corrective lens is concave lens.
29. How will you use two identical prisms so that a narrow beam of white light incident on one prism emerges out of the second prism as white light? Draw the diagram.
Ans. By placing a second identical prism in an inverted position with respect to the first prism, we get a narrow beam of white light incident on one prism emerges out of the second prism as white light.
30. Stars twinkle while the planets do not. Why?
Ans. Since the stars are very distant and the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates so stars twinkling. The planets are much closer to the earth and are thus seen as extended sources and do not twinkling.
31.What is the scattering of light? Explain with the help of an example?
Ans. When a beam of light strikes fine particles present in the atmosphere , the path of the beam becomes visible and this phenomenon is known as the scattering of light. Tyndall effect can also be observed when sunlight passes through a canopy of a dense forest.
Ans. (i) The answer is incorrect.
Reason: According to VIBGYOR word 1 is for Red and 3 is for Yellow.
(ii) Colour of Copper sulphate - blue and Danger colour is - Red.
(ii) The statement is incorrect.
Reason: The colour of chilli is red and colour of brinjal is violate. Red light bends the least while the violet the most.
Ans. The power of accommodation of the eye usually decreases with ageing. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.
25. Draw ray diagram each show:
(i) Myopic eye (ii) Hypermetropic eye.
Ans. (i) Myopic eye-
(ii) Hypermetropic eye-
26. What is meant by the near point and far point of an eye? State their values for the normal human eyes.
Ans. The minimum distance, at which objects can be seen most distinctly without strain, is called the near point of the eye. For a young adult with normal vision, the near point is about 25 cm. The farthest point up to which the eye can see objects clearly is called the far point of the eye. It is infinity for a normal eye.
27. How is normal eye able to see distinctly distances as well as near objects?
Ans. The eye lens's curvature can be modified to some extent by the ciliary muscles. When the muscles are relaxed, the lens becomes thin. Thus, its focal length increases. This enables us to see distant objects clearly.
When we are looking at objects closer to the eye, the ciliary muscles contract. This increases the curvature of the eye lens and the focal length of the eye lens decreases. This enables us to see nearby objects clearly.
28. A person needs a lens of power - 4.5D for correction of her vision.
(i) What kind of defect in visual is she suffering from?
(ii) What is the focal length of the corrective lens?
(iii) What is the nature of the corrective lens?
Ans. (i) She is suffering from myopia.
(b) Given: Power P = - 4.5D
We have P = 1/f
f = 1/Pm.
f= 1/-4.5m
f= - 1000/45 cm= -22.2cm.
(iii) Corrective lens is concave lens.
29. How will you use two identical prisms so that a narrow beam of white light incident on one prism emerges out of the second prism as white light? Draw the diagram.
Ans. By placing a second identical prism in an inverted position with respect to the first prism, we get a narrow beam of white light incident on one prism emerges out of the second prism as white light.
30. Stars twinkle while the planets do not. Why?
Ans. Since the stars are very distant and the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates so stars twinkling. The planets are much closer to the earth and are thus seen as extended sources and do not twinkling.
31.What is the scattering of light? Explain with the help of an example?
Ans. When a beam of light strikes fine particles present in the atmosphere , the path of the beam becomes visible and this phenomenon is known as the scattering of light. Tyndall effect can also be observed when sunlight passes through a canopy of a dense forest.
32. A beam of white light falling on a glass prism gets split up into seven colours marked 1 to 7 from the top. A student makes the statement:
(i) The colour at the position marked 1 and 3 are similar to the colour of turmeric and the colour of chilly powder respectively. Is the above statement correct or incorrect? Justify.
(ii) Which two positions correspond to the colour of the solution of copper sulphate and signal used to move the vehicles?
(iii) Light of colour of chilly powder bends the most while the light of colour of brinjal bends the least. Is the statement correct? Justify.Ans. (i) The answer is incorrect.
Reason: According to VIBGYOR word 1 is for Red and 3 is for Yellow.
(ii) Colour of Copper sulphate - blue and Danger colour is - Red.
(ii) The statement is incorrect.
Reason: The colour of chilli is red and colour of brinjal is violate. Red light bends the least while the violet the most.
III. Long answer type questions:
1. A student suffering from myopia is not able to see distinctly the objects placed beyond 5 m. List two possible reasons due to which this defect of vision may have arisen. With the help of ray diagrams, explain.
Ans. Myopia is known as short-sightedness. A myopic person can see nearby objects clearly but cannot see distant objects distinctly. In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself.
This defect may arise due to;
(i) excessive curvature of the eye lens, or (ii) elongation of the eyeball.
This defect can be corrected by using a concave lens of suitable power. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.
2. (i) Why the student is unable to see distinctly the objects placed beyond 5m from his eyes.
(ii) the type of corrective lens used to restore proper vision and how this defect is corrected by the use of this lens.
Ans. See the answer of Q.1
3. List the parts of the human eye that control the amount of light entering into it. Explain how they perform this function.
Ans. Iris and pupil are the two parts of the eye that controls the amount of light entering into it. Iris behind the cornea is a dark muscular diaphragm that controls the size of the pupil. The pupil regulates and controls the amount of light entering the eye.
The pupil of an eye acts like a variable aperture whose size can be varied with the help of the iris. When the light is very bright, the iris contracts the pupil to allow less light to enter the eye. However, in dim light, the iris expands the pupil to allow more light to enter the eye. Thus, the pupil opens completely through the relaxation of the iris.
4. Write the function of the retina in the human eye. Do you know that corneal impairment can be cured by replacing the defective cornea with the cornea of a donated eye? How and why should we organise groups to motivate the community members to donate their eyes after death?
Ans. The retina of human eye act as a screen. The eye lens forms an inverted real image of the object on the retina. The retina is a delicate membrane with having an enormous number of light-sensitive cells. The light-sensitive cells get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals, and finally, processes the information so that we perceive objects as they are.
By donating our eyes after we die, we can light the life of a blind person.
About 35 million people in the developing world are blind and most of them can be cured. About 4.5 million people with corneal blindness can be cured through corneal transplantation of donated eyes. One pair of eyes gives vision to TWO CORNEAL BLIND PEOPLE.
5. List three common refractive defects of vision. Suggest the way of correcting these defects.
Ans. Three common refractive defects of vision are myopia or short-sightedness, hypermetropia or long-sightedness and presbyopia.
Myopia - A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. In a myopic eye, the image of a distant object is formed in front of the retina. This defect can be corrected by using a concave lens of suitable power. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.
Hypermetropia - Hypermetropia is also known as far-sightedness. A person with hypermetropia can see distant object clearly but cannot see nearby objects distinctly. This is because the light rays from a close-by object are focussed at a point behind the retina.
This defect can be corrected by using a convex lens of appropriate power. Eye-glasses with converging lenses provide the additional focusing power required for forming the image on the retina.
Presbyopia- The power of accommodation of the eye usually decreases with ageing. They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect is called Presbyopia. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. Such people require A common type of bi-focal lenses consists of both concave and convex lenses. These days, it is possible to correct the refractive defects with contact lenses or through surgical interventions.
6. About 45 lakh people in the developing countries are suffering from corneal blindness about 3 lakh children below the age of 12 suffering from this defect can be cured by replacing the defective, with the cornea of a donated eye. How and why can a student of your age involve themselves to create awareness about this fact among people?
Ans. Try to yourself.
7. A person cannot read a newspaper place near 50 cm from his eye. Name the defect of vision he is suffering from? Draw a ray diagram to illustrate the defects. List two possible causes. Draw a ray diagram to show how this defect may be corrected using a lens of appropriate focal length. We see an advertisement for eye donation on television or a newspaper. Write the importance of such advertisement.
Ans. A person cannot read a newspaper place near 50 cm from his eye. The person is hypermetropic. He can see distant objects clearly but cannot see nearby objects distinctly. This is because the light rays from a close-by object are focussed at a point behind the retina . This defect arises either because
(i) the focal length of the eye lens is too long, or
(ii) the eyeball has become too small.
This defect can be corrected by using a convex lens of appropriate power. Eye-glasses with converging lenses provide the additional focusing power required for forming the image on the retina.
Advertisement for eye donation on television or a newspaper helps to blind people around us and more people can aware of this noble cause.
8. (a) What type of spectacles should be worn by a person having the defect of myopia as well as hypermetropia.
(b) The far point of a myopic person is 150 cm. What is the nature and the power of the lens required to correct the defect?
(c) With the help of a ray, a diagram showing the formation of image by:
(i) a myopic eye
(ii) Correction of myopia by using an appropriate lens.
Ans. (a) Spectacles of the concave lens or diverging lens should be worn by a person having the defect of myopia and converging lens for hypermetropia.
(b) The far point of a myopic person is 150 cm. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. A person with this defect has a far point nearer than infinity. A concave lens or diverging lens of suitable - ve power will bring the image back on to the retina and thus the defect is corrected.
9. A person's image when seen through a stream of hot air rising above a fire disappeared to waver. Explain.
Ans. The apparent random wavering of objects seen through a stream of hot air rising above a fire or a radiator because the air just above the fire becomes hotter than the air further up.
The hotter air is lighter or less dense than the cooler air above it, and has a refractive index slightly less than that of the cooler air. Since the physical conditions of the refracting medium are not stationary, the apparent position of the object, as seen through the hot air, fluctuates. This wavering is thus an effect of atmospheric refraction on a small scale in our local environment.
10. (a) Describe an activity along with a level diagram of the phenomenon of dispersion through a Prism.
(b) Explain in brief the formation of the rainbow with the help of the figure.
Ans. (a) Activity - Take a thick sheet of cardboard and make a small hole or narrow slit in its middle.
Allow sunlight to fall on the narrow slit. This gives a narrow beam of white light.
Now, take a glass prism and allow the light from the slit to fall on one of its faces.
Turn the prism slowly until the light that comes out of it appears on a nearby screen.
We will find a beautiful band of colours due to the dispersion of light.
(b) Formation of a rainbow- A rainbow is a natural spectrum appearing in the sky after a rain shower, formed in a direction opposite to that of the Sun. It is caused by the dispersion of sunlight by tiny water droplets, present in the atmosphere that acts like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop. Due to the dispersion of light and internal reflection, different colours reach the observer’s eye. We can also see a rainbow on a sunny day when we look at the sky through a waterfall or through a water fountain, with the Sun behind us.
11. Draw diagram of an experimental arrangement of observing scattering of light in Colloidal solution. Name the two chemicals used in this activity.
Ans. Diagram of scattering of light in colloidal solution:
Activity: . Place a strong source (S) of white light at the focus of a converging lens (L1). that provides a parallel beam of light.
. Allow the light beam to pass through a transparent glass tank (T) containing clear water.
. Allow the beam of light to pass through a circular hole (c) made in cardboard. Obtain a sharp image of the circular hole on a screen (MN) using a second converging lens (L2).
.Dissolve about 200 g of sodium thiosulphate in about 2 L of clean water taken in the tank.
Add about 1 to 2 mL of concentrated sulphuric acid to the water.
We can observe the blue light from the three sides of the glass tank that is due to scattering of short sulphur particles. The colour of the transmitted light from the fourth side of the glass tank facing the circular hole, at first the orange red colour and then bright crimson red colour on the screen.
Two chemicals used in this activity are sodium thiosulphate and sulphuric acid.
12. (I) Define dispersion. How does a prism disappear white light? Which colour of light bends the most and the least?
(II) A narrow beam of white light is passing through a glass prism. Trace it on your answer sheet and show the path of the emergent beam as observed on the screen.
(a) Write the name and the cause of the phenomenon observed.
(b) Where else in nature in this phenomenon observed.
(c) Base on the observation, state the conclusions which can be drawn about the constitution of white light.
Ans. (I) The splitting of light into its component colours is called dispersion.
White light is dispersed into its seven-colour components by a prism. Different colours of light bend through different angles with respect to the incident ray, as they pass through a prism. It is due to different wavelengths of different colour.
The red light bends the least while the violet the most.
(II)
(a) The phenomenon is a dispersion of light and it caused due to different colours of light bend through different angles with respect to the incident ray as they have different wavelengths.
(b) Rainbow after rain.
(c) The prism has probably split the incident white light into a band of seven colours. The sequence of colours are Violet, Indigo, Blue, Green, Yellow, Orange, and Red. (VIBGYOR)
13. State the natural phenomenon behind the formation of the rainbow? Explain the phenomenon. Name a device that can be used to observe such a phenomenon in the laboratory? If you are facing a rainbow in the sky, what is the position of the sun with respect to your position?
Ans. The natural phenomenon behind the formation of rainbow is dispersion.
The splitting of light into its component colours is called dispersion.
White light is dispersed into its seven-colour components by a prism. Different colours of light bend through different angles with respect to the incident ray, as they pass through a prism. It is due to different wavelengths of different colour.
The red light bends the least while the violet the most.
Prism is used to observe dispersion in the laboratory.
A rainbow is always formed in a direction opposite to that of the Sun. Therefore the position of the Sun behind me.
14. An old person is unable to see clearly nearby objects as well as distinct objects.
(a) What defect of vision is the suffering from?
(b) What kind of lens will be required to see clearly the nearby as well as distant objects? Give reasons.
Ans. (a) The defect of vision is Presbyopia in which he finds it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect is called Presbyopia.
(b) A common type of bi-focal lenses consists of both concave and convex lenses.
Reason: It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. So a person may suffer from both myopia and hypermetropia. A common type of bi-focal lenses consists of both concave and convex lenses. The upper portion consists of a concave lens. It facilitates a distant vision. The lower part is a convex lens. It facilitates near vision.
1. A student suffering from myopia is not able to see distinctly the objects placed beyond 5 m. List two possible reasons due to which this defect of vision may have arisen. With the help of ray diagrams, explain.
Ans. Myopia is known as short-sightedness. A myopic person can see nearby objects clearly but cannot see distant objects distinctly. In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself.
This defect may arise due to;
(i) excessive curvature of the eye lens, or (ii) elongation of the eyeball.
This defect can be corrected by using a concave lens of suitable power. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.
2. (i) Why the student is unable to see distinctly the objects placed beyond 5m from his eyes.
(ii) the type of corrective lens used to restore proper vision and how this defect is corrected by the use of this lens.
Ans. See the answer of Q.1
3. List the parts of the human eye that control the amount of light entering into it. Explain how they perform this function.
Ans. Iris and pupil are the two parts of the eye that controls the amount of light entering into it. Iris behind the cornea is a dark muscular diaphragm that controls the size of the pupil. The pupil regulates and controls the amount of light entering the eye.
The pupil of an eye acts like a variable aperture whose size can be varied with the help of the iris. When the light is very bright, the iris contracts the pupil to allow less light to enter the eye. However, in dim light, the iris expands the pupil to allow more light to enter the eye. Thus, the pupil opens completely through the relaxation of the iris.
4. Write the function of the retina in the human eye. Do you know that corneal impairment can be cured by replacing the defective cornea with the cornea of a donated eye? How and why should we organise groups to motivate the community members to donate their eyes after death?
Ans. The retina of human eye act as a screen. The eye lens forms an inverted real image of the object on the retina. The retina is a delicate membrane with having an enormous number of light-sensitive cells. The light-sensitive cells get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals, and finally, processes the information so that we perceive objects as they are.
By donating our eyes after we die, we can light the life of a blind person.
About 35 million people in the developing world are blind and most of them can be cured. About 4.5 million people with corneal blindness can be cured through corneal transplantation of donated eyes. One pair of eyes gives vision to TWO CORNEAL BLIND PEOPLE.
5. List three common refractive defects of vision. Suggest the way of correcting these defects.
Ans. Three common refractive defects of vision are myopia or short-sightedness, hypermetropia or long-sightedness and presbyopia.
Myopia - A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. In a myopic eye, the image of a distant object is formed in front of the retina. This defect can be corrected by using a concave lens of suitable power. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.
Hypermetropia - Hypermetropia is also known as far-sightedness. A person with hypermetropia can see distant object clearly but cannot see nearby objects distinctly. This is because the light rays from a close-by object are focussed at a point behind the retina.
This defect can be corrected by using a convex lens of appropriate power. Eye-glasses with converging lenses provide the additional focusing power required for forming the image on the retina.
Presbyopia- The power of accommodation of the eye usually decreases with ageing. They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect is called Presbyopia. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. Such people require A common type of bi-focal lenses consists of both concave and convex lenses. These days, it is possible to correct the refractive defects with contact lenses or through surgical interventions.
6. About 45 lakh people in the developing countries are suffering from corneal blindness about 3 lakh children below the age of 12 suffering from this defect can be cured by replacing the defective, with the cornea of a donated eye. How and why can a student of your age involve themselves to create awareness about this fact among people?
Ans. Try to yourself.
7. A person cannot read a newspaper place near 50 cm from his eye. Name the defect of vision he is suffering from? Draw a ray diagram to illustrate the defects. List two possible causes. Draw a ray diagram to show how this defect may be corrected using a lens of appropriate focal length. We see an advertisement for eye donation on television or a newspaper. Write the importance of such advertisement.
Ans. A person cannot read a newspaper place near 50 cm from his eye. The person is hypermetropic. He can see distant objects clearly but cannot see nearby objects distinctly. This is because the light rays from a close-by object are focussed at a point behind the retina . This defect arises either because
(i) the focal length of the eye lens is too long, or
(ii) the eyeball has become too small.
This defect can be corrected by using a convex lens of appropriate power. Eye-glasses with converging lenses provide the additional focusing power required for forming the image on the retina.
Advertisement for eye donation on television or a newspaper helps to blind people around us and more people can aware of this noble cause.
8. (a) What type of spectacles should be worn by a person having the defect of myopia as well as hypermetropia.
(b) The far point of a myopic person is 150 cm. What is the nature and the power of the lens required to correct the defect?
(c) With the help of a ray, a diagram showing the formation of image by:
(i) a myopic eye
(ii) Correction of myopia by using an appropriate lens.
Ans. (a) Spectacles of the concave lens or diverging lens should be worn by a person having the defect of myopia and converging lens for hypermetropia.
(b) The far point of a myopic person is 150 cm. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. A person with this defect has a far point nearer than infinity. A concave lens or diverging lens of suitable - ve power will bring the image back on to the retina and thus the defect is corrected.
9. A person's image when seen through a stream of hot air rising above a fire disappeared to waver. Explain.
Ans. The apparent random wavering of objects seen through a stream of hot air rising above a fire or a radiator because the air just above the fire becomes hotter than the air further up.
The hotter air is lighter or less dense than the cooler air above it, and has a refractive index slightly less than that of the cooler air. Since the physical conditions of the refracting medium are not stationary, the apparent position of the object, as seen through the hot air, fluctuates. This wavering is thus an effect of atmospheric refraction on a small scale in our local environment.
10. (a) Describe an activity along with a level diagram of the phenomenon of dispersion through a Prism.
(b) Explain in brief the formation of the rainbow with the help of the figure.
Ans. (a) Activity - Take a thick sheet of cardboard and make a small hole or narrow slit in its middle.
Allow sunlight to fall on the narrow slit. This gives a narrow beam of white light.
Now, take a glass prism and allow the light from the slit to fall on one of its faces.
Turn the prism slowly until the light that comes out of it appears on a nearby screen.
We will find a beautiful band of colours due to the dispersion of light.
11. Draw diagram of an experimental arrangement of observing scattering of light in Colloidal solution. Name the two chemicals used in this activity.
Ans. Diagram of scattering of light in colloidal solution:
Activity: . Place a strong source (S) of white light at the focus of a converging lens (L1). that provides a parallel beam of light.
. Allow the light beam to pass through a transparent glass tank (T) containing clear water.
. Allow the beam of light to pass through a circular hole (c) made in cardboard. Obtain a sharp image of the circular hole on a screen (MN) using a second converging lens (L2).
.Dissolve about 200 g of sodium thiosulphate in about 2 L of clean water taken in the tank.
Add about 1 to 2 mL of concentrated sulphuric acid to the water.
We can observe the blue light from the three sides of the glass tank that is due to scattering of short sulphur particles. The colour of the transmitted light from the fourth side of the glass tank facing the circular hole, at first the orange red colour and then bright crimson red colour on the screen.
Two chemicals used in this activity are sodium thiosulphate and sulphuric acid.
12. (I) Define dispersion. How does a prism disappear white light? Which colour of light bends the most and the least?
(II) A narrow beam of white light is passing through a glass prism. Trace it on your answer sheet and show the path of the emergent beam as observed on the screen.
(a) Write the name and the cause of the phenomenon observed.
(b) Where else in nature in this phenomenon observed.
(c) Base on the observation, state the conclusions which can be drawn about the constitution of white light.
Ans. (I) The splitting of light into its component colours is called dispersion.
White light is dispersed into its seven-colour components by a prism. Different colours of light bend through different angles with respect to the incident ray, as they pass through a prism. It is due to different wavelengths of different colour.
The red light bends the least while the violet the most.
(II)
(a) The phenomenon is a dispersion of light and it caused due to different colours of light bend through different angles with respect to the incident ray as they have different wavelengths.
(b) Rainbow after rain.
(c) The prism has probably split the incident white light into a band of seven colours. The sequence of colours are Violet, Indigo, Blue, Green, Yellow, Orange, and Red. (VIBGYOR)
13. State the natural phenomenon behind the formation of the rainbow? Explain the phenomenon. Name a device that can be used to observe such a phenomenon in the laboratory? If you are facing a rainbow in the sky, what is the position of the sun with respect to your position?
Ans. The natural phenomenon behind the formation of rainbow is dispersion.
The splitting of light into its component colours is called dispersion.
White light is dispersed into its seven-colour components by a prism. Different colours of light bend through different angles with respect to the incident ray, as they pass through a prism. It is due to different wavelengths of different colour.
The red light bends the least while the violet the most.
Prism is used to observe dispersion in the laboratory.
A rainbow is always formed in a direction opposite to that of the Sun. Therefore the position of the Sun behind me.
14. An old person is unable to see clearly nearby objects as well as distinct objects.
(a) What defect of vision is the suffering from?
(b) What kind of lens will be required to see clearly the nearby as well as distant objects? Give reasons.
Ans. (a) The defect of vision is Presbyopia in which he finds it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect is called Presbyopia.
(b) A common type of bi-focal lenses consists of both concave and convex lenses.
Reason: It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. So a person may suffer from both myopia and hypermetropia. A common type of bi-focal lenses consists of both concave and convex lenses. The upper portion consists of a concave lens. It facilitates a distant vision. The lower part is a convex lens. It facilitates near vision.
15. Explain with the help of labeled diagram the cause of
(a) twinkling of stars.
(a) twinkling of stars.
(b) "Stars seem higher than they actually are"
(c) "The sky appears dark to passengers flying at very high altitude" Justify this statement with reasons.
Ans.(a) Cause of twinkling of stars - The twinkling of a star is due to the atmospheric refraction of starlight. The atmospheric refraction occurs in a medium of gradually changing the refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. This apparent position of the star is not stationary but keeps on changing slightly, since the physical conditions of the earth’s atmosphere are not stationary. Since the stars are very distant, they approximate point-sized sources of light. As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount starlight entering the eye flickers – the star sometimes appears brighter, and at some other time, fainter, which is the twinkling effect.
(b) The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth. The atmospheric refraction occurs in a medium of gradually changing the refractive index.Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. The star appears slightly higher (above) than its actual position when viewed near the horizon.
(c) The sky appears dark to the passengers flying at very high altitude because at higher altitude the density of the atmosphere is very low so the scattering of light taking place there is very less so the sky seems to be dark.
Ans.(a) Cause of twinkling of stars - The twinkling of a star is due to the atmospheric refraction of starlight. The atmospheric refraction occurs in a medium of gradually changing the refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. This apparent position of the star is not stationary but keeps on changing slightly, since the physical conditions of the earth’s atmosphere are not stationary. Since the stars are very distant, they approximate point-sized sources of light. As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount starlight entering the eye flickers – the star sometimes appears brighter, and at some other time, fainter, which is the twinkling effect.
(b) The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth. The atmospheric refraction occurs in a medium of gradually changing the refractive index.Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. The star appears slightly higher (above) than its actual position when viewed near the horizon.
(c) The sky appears dark to the passengers flying at very high altitude because at higher altitude the density of the atmosphere is very low so the scattering of light taking place there is very less so the sky seems to be dark.
