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.

Options

A. 3.4D
B. 2.5D
C. 25D
D. 1.5D

Answer

b.

Explanation

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`