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A plano-convex lens (mu = 1.5) of apertu...

A plano-convex lens `(mu = 1.5)` of aperture diameter `8 cm` has a maximum thickness of `4mm`. The focal length of the lens relative to air is

A

`+ 20 cm`

B

`+ 40 cm`

C

`+ 60 cm`

D

`+80 cm`

Text Solution

Verified by Experts

The correct Answer is:
B
`R^(2) = (R - t)^(2) + r^(2)`
`R^(2) = R^(2) + t^(2) - 2Rt + r^(2)`
`r^(2) = 2Rt (t^(2) "is neglected")`
`R = (r^(2))/(2t) = (4 xx 4)/(2 xx 0.4) = 20 cm`
`R = 20 cm`
`:, (1)/(f) = (1.5 - 1) ((1)/(oo) -(1)/(-20))`
`(1)/(f) = (.05)/(20) :. f = + 40 cm`
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