Question

The radii of curvature of the faces of a double convex lens are $10\, cm$ and $15\, cm .$ Its focal length is $12\, cm .$ The refractive index of the glass is

• A. 1.2
• B. 1.3
• C. 1.4
• D. 1.5

Answer 1

Answer: (D)

$\frac{1}{f}=(\mu-1)\left[\frac{1}{R_{1}}-\frac{1}{R_{2}}\right]$
$\Rightarrow (\mu-1)\left[\frac{1}{+10}-\frac{1}{(-15)}\right]$
$=\frac{1}{12}=(\mu-1)\left(\frac{1}{10}+\frac{1}{15}\right)=\frac{1}{12}$
$\Rightarrow \mu=1.5$