Question

The radius of curvature of the convex surface of a plano-convex lens is $10cm$. What is the focal length of the plano-convex lens? (Here $\mu$ = $1.5$)

• A. $10cm$
• B. $20cm$
• C. $15cm$
• D. $5cm$

Answer 1

The correct option is B $20cm$

$\mu =1.5,R_1=10cm,R_2=0$

As focal length is given by $\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$ (lens maker formula)

$\frac{1}{f}=0.5\times (\frac{1}{10}-0)$

$f=20cm$

hence, the focal length of the plano-convex lens is $20cm$.