Question

A double convex lens has focal length 25.0 cm. The radius of one of the surfaces is double of the
other. Find the radii of curvature if the refractive index of the material of the lens is 1.5.

Answer 1

Given :

f=20cm

Let us consider = R1=R

R2= 2R1= - 2R[ sign conventions]

n=1.5

From lens maker's formula:

1/f= (n-1)[1/R1- 1/R2]

1/f=( 1.5-1)[1/R+1/2R]

1/f=0.5[2+1/2R]

1/f= 5x3/10 x2R

1/f= 3/4R

f= 4R/3

R=3x25/4

R1=R=18.75cm

R2= 2R1= 2x18.75 =37.5cm