Question

A concave lens of glass, refractive index 1.5 has both surfaces of same
radius of
curvature R. On immersion in a medium of refractive index 1.75, it will
behave as a​

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Ray Optics and Optical Instruments

Combination of Mirror and Lenses

A concave lens of glass, re...

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Asked on November 22, 2019 by

Bijayalaxmi Acharya

A concave lens of glass, refractive index 1.5, has both surfaces of same radius of curvature R. On immersion in a medium of refractive index 1.75, it will behave as a :

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ANSWER

Consider refraction of light from infinity at first surface,

v

μ

2

−

u

μ

1

=

R

μ

2

−μ

1

v

1

1.5

−

−∞

1.75

=

−R

1.5−1.75

Consider refraction of this image from second surface,

v

2

1.75

−

v

1

1.5

=

R

1.75−1.5

Hence, v

2

=3.5R

This is the image of light coming from infinity, therefore the focal length.

It is converging since focal length is positive