Question

A biconvex lens in air has radii of curvature $20cm$ each. If the refractive index of the material of the lens is $1.5$, what is its focal length?

• A. $20cm$
• B. $-10cm$
• C. $10cm$
• D. $-20cm$

Answer 1

The correct option is A $20cm$

Given:
Refractive index of the lens (with respect to air), $n=1.5$
$R_1=20cm$
$R_2=-20cm$.
(According to sign convention)

From Lens Maker's formula,
$\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$
$\frac{1}{f}=(1.5-1)(\frac{1}{20}-\frac{1}{(-20)})$
$f=20cm$