Question

The radii of curvature of a double convex lens are $15cm$ and $30cm$ and the refractive index of its glass is $1.5$. Calculate the focal length of the lens.

• A. $10cm$
• B. $30cm$
• C. $25cm$
• D. $20cm$

Answer 1

The correct option is D $20cm$

$\begin{array}{c}{R}_1=15cm\\ R_2=-30cm\\ \mu =1.5\end{array}$

$\begin{array}{cc}& \text{Using lens Maker's formula}\\ \frac{1}{f}& =(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})\\ \frac{1}{f}& =(1.5-1)(\frac{1}{15}+\frac{1}{30})\\ & =0.5\left(\frac{2+1}{30}\right)\\ \frac{1}{f}& =\frac{0.5\times 3}{300}\\ f& =20cm\end{array}$