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,

`(1)/(f)=(1)/(i)+(1)/(o)`

On rearranging the above equation,

`(1)/(i)=(1)/(f)-(1)/(o)`

On substituting the known values,

`\begin{gathered} (1)/(i)=(1)/(-39)-(1)/(30) \\ =-0.06 \\ i=(1)/(-0.06) \\ =-16.67\text{ cm} \end{gathered}`

Thus the image distance is -16.67 cm