Question

A concave lens has a focal length of -39 cm.Find the image distance that results when an object is placed 30 cm in front of the lens.

Answer 1

Given,

The focal length of the lens, f=-39 cm

The object distance, o=30 cm

From the thin lens formula,

$\frac{1}{f}=\frac{1}{i}+\frac{1}{o}$

On rearranging the above equation,

$\frac{1}{i}=\frac{1}{f}-\frac{1}{o}$

On substituting the known values,

$\begin{gathered} \frac{1}{i}=\frac{1}{-39}-\frac{1}{30} \\ =-0.06 \\ i=\frac{1}{-0.06} \\ =-16.67\text{ cm} \end{gathered}$

Thus the image distance is -16.67 cm