First slide
Ray optics

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 


120=1.5μ12R     and     110=1.512R=1RR=10cm

So,  μ=2

t=10  secs

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