Question

The radii of curvature of both the sides of a convex lens are $15cm$ and if the refractive index of material of the lens is $1.5$. Then focal length of lens in air is _______cm.

• A. $10$
• B. $20$
• C. $15$
• D. $30$

Answer 1

The correct option is C $15$

According to the len's maker formula,

$\frac{1}{f}=(\frac{{\mu }_l}{{\mu }_m}-1)(\frac{1}{R_1}-\frac{1}{R_2})\frac{1}{f}=(\frac{\frac{3}{2}}{1}-1)(\frac{1}{15}-(-\frac{1}{15}))\frac{1}{f}=\left(\frac{1}{2}\right)\left(\frac{2}{15}\right)\therefore f=15cm$