Q.

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

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a

5 secs

b

10 secs

c

20 secs

d

25 secs

answer is 2.

(Detailed Solution Below)

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Detailed Solution

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