Question

An object is placed at a distance of 4 cm from a concave lens of focal length 12 cm. Fine the position and nature of the image. 

Answer 1

Object distance u = -4 cm (left side of the lens)
Focal length f = -12 cm (left side of the lens)
Image distance v  = ?
From the lens formula, we know that:  

`1/v-1/u=1/f` 

Substituting the values of v and u, we get:     

`1/v-1/u=1/f` 

⇒`1/v-1/-4=1/-12` 

⇒`1/v=-1/12-1/4` 

⇒`1/v=(-3-1)/12` 

⇒`1/v=(-4)/12` 

⇒`v=12/-4` 

⇒`v=-3` cm  

Thus, the image is virtual and formed 3 cm away from the lens, on the left side.