Question

The radii of curvature of a double concave lens are $30cm$ and $60cm$ and the refractive index of its glass is $1.5$. Calculate the focal length of the lens.

Answer 1

we know that distance measured left is taken negative from o and rightward is taken positive conventionally.
So, $R_1=-30cm$ and $R_2=60cm$
$\begin{array}{c}\frac{1}{f}=(\mu -1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)\\ =\left(\frac{3}{2}-1\right)\left(\frac{-1}{30}-\frac{1}{60}\right)\\ =\frac{1}{2}\times \frac{-3}{60}=\frac{-1}{40}\\ so,f=-40\end{array}$