Question

A plastic plano-concave lens has a radius of curvature of 58 cm for its concave surface. If the index of refraction of the plastic is 1.25, what is the power of the lens?

Answer #1

**Solution:**

Let us go to the basics first.

The lens maker's formula is:

1/f =( n-1){(1/R1) - (1/R2)},

where n is the refractive index of the material of the lens with respect to the medium in which it is placed, R1, R2 are radii of curvature of the two surfaces of the lens.

Given that the refractive index of material of the lens = 1.25, and R1 =58 cm and R2= infinity ( for the plane surface),

Substituting, these values we get,

1/f =( 1.25-1){(1/58) - 0},

=>f =232 cm =232/100 m =2.32 m

**Therefore power of the lens**=1/2.32 **=
0.431 diopter
(Answer)**

**Thanks!!!**

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