Question

A spherical convex surface separates object and image spaces of refractive indices 1.0 and `4//3` . If radius of curvature of the surface is 10cm, find its power.
A. 3.4D
B. 2.5D
C. 25D
D. 1.5D

Answer 1

Correct Answer - b.
Let us see where does the parallel rays converge or diverge on the principal axis. Let us call it the focus and the corresponding length the focal length f. Using
`(mu_(1))/(v)-(mu_(2))/(u)=(mu_(2)-mu_(1))/(R)`
With proper values and signs, we have
`(4//3)/(f)-(1.0)/(oo)=(4//2-1.0)/(+10)`
or `f=40cm =0.4m`
Since the rays are converging, its power should be positive.
Hence,
P(in dioptre) `=(+1)/(f("meter"))=(1)/(0.4)`
or `P=2.5D`