Question

Given $ _{a}{{\mu }_{g}}=3/2 $ and $ _{a}{{\mu }_{w}}=4/3 $ . There is an equiconvex lens with radius of each surface equal to $20\, cm$. There is air in the object space and water in the image space. The focal length of lens is

• A. 80 cm
• B. 40 cm
• C. 20 cm
• D. 10 cm

Answer 1

Answer: (B)

$\frac{a \mu_{i}}{f}=\frac{\left(a_{a}-1\right)}{R_{1}}-\frac{\left(\mu_{g}-a \mu_{w}\right)}{R_{2}}$
$=\frac{\left(\frac{3}{2}-1\right)}{20}-\frac{\left(\frac{3}{2}-\frac{4}{3}\right)}{-20}$
$=\frac{1}{40}+\frac{1}{120}=\frac{1}{30}$
$\therefore f=\frac{4}{3} \times 30=40\, cm$