Question

A plano-concave lens is made of glass which has a refractive index of 1.5. One surface is flat, and the concave surface has a radius of curvature of 25 cm. determine the focal length of this lens.

Answer 1

We know that

$\frac{1}{f}=(\mu-1)(\frac{1}{R_2}-\frac{1}{R_2})$

$\frac{1}{f}=(1.5-1)(\frac{1}{infinite}-\frac{1}{25})$

$\frac{1}{f}=0.5(0-\frac{1}{25})$

$\frac{1}{f}=-\frac{1}{50}$

$f=-50 cm$