Question

A double convex lens has two surfaces of equal radii $15cm$ and refractive index $\mu =1.5$, its focal length is equal to

• A. -15 cm
• B. -30 cm
• C. +15 cm
• D. +30 cm

Answer 1

Answer: (C)

$\frac{1}{f}=(\mu -1)\left(\frac{1}{R}-\frac{1}{-R}\right)$
$\frac{1}{f}=0.5\left(\frac{2}{15}\right)$
$\therefore f=15cm$