Question

The refractive index of the material of a double convex lens is $1.5$ and its focal length is $5\, cm$. If the radii of curvature are equal, the value of the radius of curvature (in $cm$ ) is

• A. 5.0
• B. 6.5
• C. 8.0
• D. 9.5

Answer 1

Answer: (A)

$\frac{1}{f}=(\mu-1)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$
For double convex lens, $R_{1}=R,\, R_{2}=-R$
$\therefore \frac{1}{5}=(1.5-1)\left(\frac{1}{R}+\frac{1}{R}\right)$
or $\frac{1}{5}=0.5 \times \frac{2}{R}$
or $R=5\, cm$