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Questions  

An equiconvex lens of focal length 10cm (at t=0)  and refractive index (1.5)  is placed in a medium whose refractive index varies with time as μ=1+t10;  t in seconds. The  value of t when the lens acts as a diverging lens of focal length 20cm is 

a
5 secs
b
10 secs
c
20 secs
d
25 secs

detailed solution

Correct option is B

−120=1.5μ−12R     and     110=1.5−12R=1R⇒∴R=10cmSo,  μ=2∴t=10  secs

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