Question

If the focal length of a double convex lens is $12cm$ and radii of curvatures of faces are $10cm$ and $15cm$ respectively, what is the refractive index of the lens?

Answer 1

Given - $f=12cm,R_1=+10cm,R_2=-15cm$

The focal length of the lens is given by ,

$\frac{1}{f}=(n-1)(\frac{1}{R_1}-\frac{1}{R_2})$

or $\frac{1}{12}=(n-1)(\frac{1}{10}-\frac{1}{-15})$

or $\frac{1}{12}=(n-1)\left(\frac{1}{6}\right)$

or $(n-1)=1/2$

or $n=3/2=1.5$