A plano-convex lens has diameter 5 cm and its thickness at the centre is 0. 25 cm. If the speed of light inside the lens is 2 x 10 8 ms -1 , then what is the focal length (in cm) of the lens?
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25.25
R. I. of lens,
From 
ACO, radius of curvature of lens is,
R 2 = 
+ (R-0.25) 2
R 2 = (2. 5) 2 + (R - 0. 25) 2
R 2 = 6. 25 + R 2 + 0.0625 - 0.5R
0.5R = 6.3125
R= 12.625cm
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