Question

An object is places at a distance 40 cm from a thin converging lens of focal length; 10cm. Calculate the image distance.

• A. $-\frac{40}{3}c.m$
• B. $\frac{40}{3}c.m$
• C. $-\frac{20}{3}c.m$
• D. $\frac{20}{3}c.m$

Answer 1

The correct option is B $\frac{40}{3}c.m$


f = 10 cm
u = -40 cm
Applying lens formula;
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$ Gives us;
$\frac{1}{v}=\frac{1}{f}+\frac{1}{u}$
$=\frac{1}{10}-\frac{1}{40}=\frac{3}{40}$
$\Rightarrow v=\frac{40}{3}c.m$