The radius of curvature of each surface of a double convex lens is 15 cm and the refractive index of the medium of the lens is 1.5. What is the focal length of the lens ?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 15 cm
$\frac{1}{f}=(n-1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$ [Len's maker formula]
Given,
$R_1=15cm$
$R_2=-15cm$
$n=1.5$
$∴\frac{1}{f}=(1.5-1)\left(\frac{1}{15}-\frac{1}{-15}\right)$
$\frac{1}{f}=\frac{1}{15}$
$f=15cm$