Question

Find the focal length of a lens made up of two identical spherical surfaces having radius of curvature $10\text{cm}$ and refractive index $1.5$.

• A. $5\text{cm}$
• B. $10\text{cm}$
• C. $20\text{cm}$
• D. None

Answer 1

The correct option is B $10\text{cm}$

For the above lens

${\mu }_1=1;{\mu }_2=1.5$

$R_1=+10\text{cm};R_2=-10\text{cm}$

Using lens maker's formula:

$\frac{1}{f}=(\frac{{\mu }_2}{{\mu }_1}-1)(\frac{1}{R_1}-\frac{1}{R_2})$

putting the value, we get

$\frac{1}{f}=(\frac{1.5}{1}-1)(\frac{1}{10}-\frac{1}{(-10)})$

$\frac{1}{f}=0.5\left(\frac{2}{10}\right)$

$\therefore f=+10\text{cm}$

Hence, option $\left(b\right)$ is correct.

Why this question ? Introduction to lens maker's formula & how it is different from the normal lens formula.