Quick Review of CH - 11 The Human Eye and the Colourful World

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CH - 11  The Human Eye and the Colourful World

Class X 

Syllabus of Eye -Functioning of a lens in the human eye, defects of vision and their corrections, applications of spherical mirrors and lenses. Refraction of light through a prism, dispersion of light, scattering of light, applications in daily life.

1. The human eye is one of the most valuable and sensitive sense organ that is capable of receiving visual images and enables us to see the wonderful world and the colours around us.
2. Retina - The human eye is like a camera and forms an image on a light-sensitive screen called the retina. It is a layer of nervous tissue and covers the inside of the back two-thirds of the eyeball. In retina stimulation by light occurs, initiating the sensation of vision.
3. Cornea - Light enters the eye through a thin transparent membrane of the front of the eye called the cornea. The cornea covers the iris and pupil.
4. Iris – It is a flat, round-shaped and colour dark muscular diaphragm behind the cornea of the eye and controls the size of the pupil.
5. Pupil -  It is a hole present in the center of the iris that regulates and controls the amount of light.
6. How can I see?--The eye lens forms an inverted diminished and real image of the object on the retina which is a delicate membrane and contains an enormous number of light-sensitive cells and these cells get activated upon illumination and generate electrical signals. Through the optic nerves of the retina, these signals are sent to the brain which interprets these signals, and finally, processes the information so that we perceive objects as they are.
7. The function of ciliary muscles - When the ciliary muscles are relaxed, the lens becomes thin and its focal length increases which enables us to see distant objects clearly. When the ciliary muscles contract and eye lens then become thicker, its focal length decreases which enables us to see the nearest objects clearly.
8. Accommodation - The ability of the eye of adjusting its focal length to focus both near and distant objects, is called the accommodation of the eye.
9. Near point - The smallest distance, at which the eye can see objects clearly and comfortably without any strain, is called the near point of the eye or the least distance of distinct vision. For a young and adult with normal vision, it is about 25 cm.
10. Cataract - At an old age, sometimes the lens of people becomes milky and cloudy. This condition is called a cataract.
11. Myopia or short-sightedness – It is an eye disorder in which the image of distant objects is focused before the retina and we can't see the clearly distant objects but we can see near objects clearly. It is corrected by using a concave lens of suitable power. The concave lens diverges the light rays and focused on the retina.
12. Hyper-metopic or far-sightedness – It is an eye disorder in which the image of nearby objects is focused beyond the retina and so we can't see the near object clearly but can see the distant objects clearly. It is corrected by using a convex lens of suitable power. The convex lens converges the light rays and focused on the retina.
13. The eye loses its power of accommodation at old age due to the weakening of the ciliary muscles and diminishing flexibility of the eye lens.
14. Presbyopia: The power of accommodation of the eye usually decreases with ageing, the near point gradually increases due to this we can’t see near objects which is difficult to see nearby objects clearly without corrective eye-glasses. This defect is called Presbyopia.  
15. Lateral displacement - For parallel refracting surfaces, as in a glass slab, the emergent ray is parallel to the incident ray and it is slightly displaced laterally and the perpendicular distance between the incident ray and emergent ray is called lateral displacement.
16. Glass prism - Prism is a transparent optical triangular glass element. It has two triangular bases and three rectangular lateral surfaces. These surfaces are inclined to each other.
17. The angle of prism: The angle between its two lateral faces is called the angle of the prism.
18. The angle of deviation - When a ray of light enters the prism and refract, the emergent ray bend at an angle to the direction of the incident ray, this angle is called the angle of deviation.
19. Dispersion - The splitting of white light into its component colours is called dispersion.
20. Spectrum -Due to dispersion, the band of the coloured components of a light beam is called its spectrum.
21. The various colours seen in the spectrum are Violet, Indigo, Blue, Green, Yellow, Orange, and red.
22. Scattering of light causes the blue colour of the sky and the red colour of the Sun during sunrise and sunset.
23. Isaac Newton was the first to use a glass prism to obtain the spectrum of sunlight.
24. White light is dispersed into its seven-color components by a prism because of different colours of light bend through different angles due to different speed of different colour with respect to the incident ray, as they pass through a prism.
25. The red light bends the least means higher in speed, while the violet the most means lesser in speed.
26. A rainbow is a natural spectrum appearing in the sky after a rain shower due to the dispersion of sunlight by tiny water droplets that acts like small prisms, present in the atmosphere. Tiny water droplets refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop. A rainbow is always formed in a direction opposite to that of the Sun.
27. Twinkling of stars: The twinkling of a star is due to atmospheric refraction of starlight. The 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 of starlight entering the eye flickers, thus the star sometimes appears brighter, and at some other time, fainter, which is the twinkling effect.
28. The planets don’t twinkle - The planets are much closer to the earth, and are thus seen as extended sources. If we consider a planet as a collection of a large number of point-sized sources of light, the total variation in the amount of light entering our eye from all the individual point-sized sources will average out to zero, thereby nullifying the twinkling effect.
29. The star appears slightly higher than its actual position: 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 than its actual position.
30. Advance sunrise and delayed sunset: The Sun is visible to us about two minutes before the actual sunrise, and about two minutes after the actual sunset because of atmospheric refraction.
31. The blue colour of the sky, colour of water in the deep sea, the reddening of the sun at sunrise and the sunset are some of the wonderful phenomena due to the scattering of light.
32. Tyndall effect - When a beam of light strikes the fine particles of the earth’s atmosphere that is a heterogeneous mixture of minute particles, the path of the beam becomes visible. The light reaches us, after being reflected diffusely by these particles. This phenomenon of scattering of light by the colloidal particles gives rise to the Tyndall effect.
33. Light from the Sun overhead would travel relatively shorter distances. At noon, the Sun appears white as only a little of the blue and violet colours are scattered.
34. 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 and red colour has a longer wavelength. This gives rise to the reddish appearance of the Sun.
35. The colour of the clear Sky Blue: The colour of the scattered light depends on the size of the scattering particles. Very fine particles scatter mainly blue light of smaller wavelengths. The molecules of air and other fine particles in the atmosphere have a size smaller than the wavelength of visible light. Thus, when sunlight passes through the atmosphere, the fine particles in the air scatter the blue colour (shorter wavelengths) more strongly than red ( longer wavelength) and the colour of the clear sky becomes blue.
36. Particles of larger size scatter light of longer wavelengths. If the size of the scattering particles is large enough, then, the scattered light may even appear white.
37. The sky appears dark to passengers flying at very high altitudes, as scattering is not prominent at such heights because the earth had no atmosphere at this height, there would not have been any scattering. Then, the sky would have looked dark.
38. ‘Danger’ signal lights are red in colour. The red has a larger wavelength, higher speed and is least scattered by fog or smoke. Therefore, it can travel long distances and can be seen in the same colour at a distance.