Class X CH – 10 Light – Reflection and Refraction
Syllabus
of Light -
Reflection of light by curved surfaces; Images formed by spherical mirrors, center
of curvature, principal axis, principal focus, focal length, mirror formula
(Derivation not required), magnification.
Refraction;
Laws of refraction, refractive index.
Refraction of
light by the spherical lens; Image formed by spherical lenses; Lens formula
(Derivation not required); Magnification. Power of a lens.
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- Mirror formula:
1/f = 1/u + 1/v
Gives the relationship between the object-distance
(u), image-distance (v), and focal length (f) of a spherical mirror.
Sign for : Concave mirror –
Object distance u = - ve, Image distance v = - ve (when object is placed between infinity to focus ) and Image distance v = + ve ( when object is placed between focus and pole) and Focus f = - ve.
Sign for: Convex mirror-
u = - ve, v = +ve and f = + ve . - Lens formula: 1/f=1/v-1/u
It gives the relationship between the object distance (u), image distance (v), and the focal length (f) of a spherical lens.
Sign for: convex lens: u = - ve , v = + ve (when object is placed between infinity to focus ) and v = - ve ( when object is placed between focus and pole) and Focus f = + ve. - Focal length - Focal length of a spherical mirror or lens is equal to half its radius of curvature. F = R/2.
- Uses of convex mirrors - Convex mirrors are commonly used as rear-view mirrors in vehicles.
Convex mirrors always give an erect, diminished image. Also, they have a wider field of view as they are curved outwards. Convex mirrors enable the driver to view a much larger area than would be possible with a plane mirror. - Uses of concave mirrors-
(i) Concave mirrors are commonly used in torches, search-lights and vehicle headlights to get powerful parallel beams of light.
(ii) They are often used as shaving mirrors to see a larger image of the face. The dentists use concave mirrors to see large images of the teeth of patients.
(iii) Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces. - The magnification produced by a plane mirror is +1
Positive (+) indicates an image formed by the mirror is virtual and erect.
ii. m = 1 indicates the size of the image is equal to the size of the object. - Light travels in a vacuum with an enormous speed of 3×108 m/s. The speed of light is different in different media.
- Refractive index -Refractive index of a transparent medium is the ratio of the speed of light in a vacuum to that in the medium. η = speed of light in vacuum/speed of light in the medium Refractive the index has no unit because it is the ratio that cancels out.
- In the case of a rectangular glass slab, the refraction takes place at both the air-glass interface and the glass-air interface. The emergent ray is parallel to the direction of the incident rays.
- Combination power P = P1 + P2 + P3 + …( If the individual powers are P1, P2, P3, …)
- Power of a lens - The degree of convergence or divergence of light rays achieved by a lens is expressed in terms of its power. The power of a lens is defined as the reciprocal of its focal length. It is represented by the letter P. The SI unit of power of a lens is diopter (D).P = 1/f (unit of ‘f’ is in m).
- The power of a convex lens is positive and that of a concave lens is negative.
- Laws of reflection:
(i) The incident ray, the reflected ray and the normal to the surface of the mirror at the point of incidence, all lie in the same plane.
(ii) The angle of incidence equals the angle of reflection. - The real image is obtained when a ray of light after reflection actually converges and intersects at a point. It can be obtained on the screen and can be seen with the eye.
- The virtual image formed when rays of light rays only appear to diverge and do not actually meet but appear to meet when produced backward. It cannot be obtained on the screen.
- Characteristics of the image formed by the plane mirror:
(i)The image formed by a plane mirror is always virtual and erect.
(ii) The size of the image equals to the size of the object and the image is laterally inverted.
(iii) The image formed by the plane mirror is far behind the mirror as the object is in the front. - Lateral inversion: The phenomenon due to which the right side of the object appears as left and the left side of the object appears as a right that is the image is inverted sideways is called lateral inversion.
- Concave mirror mostly forms a real image that can be received on the screen.
- The convex mirror always forms a virtual image that cannot be received on the screen.
- Differences between a plane mirror, a concave mirror and a convex mirror without touching them:
(i) If the image formed is erect and is of the same size as in reality then it is a plane mirror.
(ii) If the image formed is still erect but smaller in size than it is a convex mirror.
(iii) If the image formed is erect but magnified when the mirror is close to the object then it is a concave mirror. - The center of the reflecting surface of a spherical mirror is called the pole of the mirror and usually represented by P.
- The horizontal line passing through the center of curvature and pole of the spherical mirror is known as a Principal axis.
- The center of curvature of a spherical mirror is the center of the hollow sphere of glass, of which the spherical mirror is a part and is usually represented by C.
- The radius of curvature of a spherical mirror is the radius of a hollow sphere of glass, of which the spherical mirror is a part and is usually represented by R.
- The diameter of the reflecting surface that is twice the radius is called its aperture.
- Radius of curvature R = 2 x focal length (f).
- Rules for ray diagram:
(i) The path of a reflected ray depends upon how incident ray is oriented with respect to the principal axis.
(ii) A ray of light passing through the principal focus of a mirror becomes parallel to the principal axis of the mirror on reflection.
(iii) A ray of light parallel to the principal axis, after reflection, passes through the principal focus.
(iv) A ray of light incident obliquely towards the pole of the mirror is reflected obliquely as per the laws of reflection.
(v) A ray of light passing through the center of curvature of a mirror is reflected back along the same path. - The phenomenon of change in the path of light from one medium to another is called the refraction of light. The angle formed between the incident ray and the normal range is called the angle of incidence and angle between the reflected ray and the normal ray is called the angle of refraction.
- The cause of refraction is the change in the speed of light as it goes from one medium to another medium.
- Larger the differences in speed of light between the two media across the interface, the greater will be the angle of bending and vice versa.
- When a Ray of light passes from rarer medium to denser medium it bends towards the normal, also the angle of incidence is greater than the angle of refraction.
- When a Ray of light passes from denser medium to rarer medium it bends away from the normal also the angle of incidence is less than the angle of refraction.
- Laws of refraction:
First law: The incident rays the reflected ray and the normal at the point of incidence all lie in the same plane.
Second law: The ratio of the sine of the angle of incidence is the first medium to the Sine of the angle of refraction in the second medium is a constant for a given pair of medium and for a given wavelength of light.
n = sin i / sin r
Where n is a constant known as the refractive index of the second medium with respect to the first medium this law is also known as snails law. - The ratio of the speed of light in a vacuum to the speed of light in a medium is called the refractive index of the medium. It has no unit.
- The shifting of emergent ray sideways from the direction of the original incident ray is called lateral displacement. Lateral displacement is produced during the refraction of light through a glass slab.
- The extent of the lateral shift produced due to the refraction across parallel slab depends on:
(i) The angle of incidence,
(ii) The thickness of the slab and
(iii) The refractive index of the glass slab. - The angular deviation is an angle through which a ray of light deviates on passing through a prism.
- A convex lens is thicker at the middle than at the edge
- A concave lens is thicker at the edges than in the middle.
- Convex lens converges the rays of light while the concave lens diverges.
- As the object moves towards the optical center of a convex lens, the image moves away from the optical center except when the object is placed between focus and the optical center of the lens.
- The magnification produced by a spherical mirror is the ratio of the height of the image to the height of the object. Magnification is positive for virtual image and negative for real image.
For mirror the magnification (m) = height of image/ height of object
= h’/h
= - v / u
For lens the magnification (m) = height of image/ height of object
= h’/h
= + v / u . - Image formation by the concave mirror for different position of the object
Position of object
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Position of image
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Size of Image
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Nature of Image
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At infinity
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At focus
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Highly diminished
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Real and Inverted
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Beyond C
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Between F and C
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Diminished
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Real and Inverted
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At C
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At C
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Same size
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Real and Inverted
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Between C and F
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Beyond C
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Enlarged
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Real and Inverted
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At F
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At infinity
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Highly enlarged
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Real and Inverted
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Between P and F
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Behind mirror
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Enlarged
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Virtual and erect
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Image formation by the convex mirror for different position of the object
Position of object
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Position of image
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Size of Image
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Nature of Image
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At infinity
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At focus behind the mirror
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Highly diminished
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Virtual and erect
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Between infinity and pole
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Between P and F behind the mirror.
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Diminished
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Virtual and erect
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