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.