Question

A concave lens of 20 cm focal length forms an image 15 cm from the lens. Compute the object distance.

Answer 1

Focal length of concave lens (f) = −20 cm (focal length is negative for a concave lens)
Image distance (v) = −15 cm (negative sign is given because the image formed by a concave lens is on the left side of the lens)

Using the lens formula:

`1/f=1/v-1/u`

`1/-20=1/-15-1/u`

`1/u=1/20+1/((-15)`

`1/u=(3-4)/60=-1/60` 

∴  u = −60 cm

The negative sign shows that the object is on the left side of the concave lens.
Therefore, the object is at a distance of 60 cm and on the left side of the concave lens.