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Two similar thin equi-convex lenses, of ...

Two similar thin equi-convex lenses, of focal f each, are kept coaxially in contact with each other such that the focal length of the combination is `F_(1)`, When the space between the two lens is filled with glycerin (which has the same refractive index `(mu=1.5)` as that of glass) then the equivlent focal length is `F_(2)`, The ratio `F_(1): F_(2)` will be

A

`2:1`

B

`1:2`

C

`2:3`

D

`3:4`

Text Solution

Verified by Experts

The correct Answer is:
B

Equivalent focal length in air `1/(F_1) = 1/f + 1/f = 2/f`

When glycerin is filled inside, glycerin lens behaves diverging lens of focal length (-f)
`1/(F_2) = 1/f + 1/f - 1/f = 1/f`
`(F_1)/(F_2) = 1/2`
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Knowledge Check

  • Two thin lenses of focal lengths f_1 and f_2 are placed in contact with each other. The focal length of the combination is :

    A
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