Page 164 - NCERT Science Class 10 English Medium

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n Mirror formula, + = , gives the relationship between the object-distance (u),
v u f
image-distance (v), and focal length (f) of a spherical mirror.
n The focal length of a spherical mirror is equal to half its radius of curvature.
n The magnification produced by a spherical mirror is the ratio of the height of the
image to the height of the object.
n A light ray travelling obliquely from a denser medium to a rarer medium bends
away from the normal. A light ray bends towards the normal when it travels obliquely
from a rarer to a denser medium.
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n Light travels in vacuum with an enormous speed of 3×10 m s . The speed of light
is different in different media.
n The refractive index of a transparent medium is the ratio of the speed of light in
vacuum to that in the medium.
n In case of a rectangular glass slab, the refraction takes place at both air-glass
interface and glass-air interface. The emergent ray is parallel to the direction of
incident ray.
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n Lens formula, – = , gives the relationship between the object-distance (u),
v u f
image-distance (v), and the focal length (f) of a spherical lens.
n Power of a lens is the reciprocal of its focal length. The SI unit of power of a lens is
dioptre.

E X E R C I S E S

1. Which one of the following materials cannot be used to make a lens?
(a) Water (b) Glass (c) Plastic (d) Clay
2. The image formed by a concave mirror is observed to be virtual, erect and larger
than the object. Where should be the position of the object?
(a) Between the principal focus and the centre of curvature
(b) At the centre of curvature
(c) Beyond the centre of curvature
(d) Between the pole of the mirror and its principal focus.
3. Where should an object be placed in front of a convex lens to get a real image of the
size of the object?
(a) At the principal focus of the lens
(b) At twice the focal length
(c) At infinity
(d) Between the optical centre of the lens and its principal focus.
4. A spherical mirror and a thin spherical lens have each a focal length of –15 cm. The
mirror and the lens are likely to be
(a) both concave.
(b) both convex.

Light – Reflection and Refraction 159

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