State one assumption made in deriving the formula n 2 v − n 1 u = n 2 − n 1 R for refraction at a spherical interface.

While deriving the formula for refraction at a spherical interface, the assumption made is that the aperture of the spherical surface is small. In such a case the point of incidence will be close to the pole of the spherical surface due to which the angle of incidence and angle of refraction will be small too. Hence, in radian, sin i = i and sin r = r which is one of the important steps in the derivation of the formula n 2 v − n 1 u = n 2 − n 1 R .

E M B I B E

ISC Physics Class 12, Part-2>Chapter 3 - Refraction of Light at Spherical Surfaces : Lenses>QUESTIONS>Q 9

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A symmetric double convex lens of power $4 D$ is cut in two halves by a plane containing the principal axis. The power of each half is:

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Important Questions on Refraction of Light at Spherical Surfaces : Lenses

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ISC Physics Class 12, Part-2>Chapter 3 - Refraction of Light at Spherical Surfaces : Lenses>QUESTIONS>Q 10

Two identical plano-convex lenses can be combined in three ways, as shown. The ratio of the focal lengths of these combinations will be:

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ISC Physics Class 12, Part-2>Chapter 3 - Refraction of Light at Spherical Surfaces : Lenses>QUESTIONS>Q 11

A convex lens of power

+ 6

dioptre is placed in contact with a concave lens of power

- 4

dioptre. What will be the nature and focal length of this combination?

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ISC Physics Class 12, Part-2>Chapter 3 - Refraction of Light at Spherical Surfaces : Lenses>QUESTIONS>Q 12

Two thin lenses having optical powers of

- 10 D

and

+ 6 D

are placed in contact with each other. The focal length of the combination is:

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ISC Physics Class 12, Part-2>Chapter 3 - Refraction of Light at Spherical Surfaces : Lenses>QUESTIONS>Q 13

A lens of power

+ 2 D

is placed in contact with a lens of power

- 1 D

. The combination will behave like:

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ISC Physics Class 12, Part-2>Chapter 3 - Refraction of Light at Spherical Surfaces : Lenses>QUESTIONS>Q 14

Two thin lenses are in contact and the focal length of the combination is

80 cm

. If the focal length of one lens is

20 cm

, then the power of the other lens will be:

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IMPORTANT

ISC Physics Class 12, Part-2>Chapter 3 - Refraction of Light at Spherical Surfaces : Lenses>QUESTIONS>Q 1

State one assumption made in deriving the formula

\frac{n_{2}}{v} - \frac{n_{1}}{u} = \frac{n_{2} - n_{1}}{R}

for refraction at a spherical interface.

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IMPORTANT

ISC Physics Class 12, Part-2>Chapter 3 - Refraction of Light at Spherical Surfaces : Lenses>QUESTIONS>Q 2

Can the optical centre of a lens be situated outside the lens?

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ISC Physics Class 12, Part-2>Chapter 3 - Refraction of Light at Spherical Surfaces : Lenses>QUESTIONS>Q 3

A thin convex lens $(L_{1})$ of focal length $80 cm$ and a thin concave lens $(L_{2})$ of focal length $f$ are kept co-axially $20 cm$ apart as shown in figure below. When a narrow and parallel beam of light is incident on the convex lens, beam emerging from the concave lens is also a parallel beam. Find $f$ .