Question

A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.

Answer 1

this will be the diagram and the solving portion is.

Answer 2

focal length, f = -15 cm

Distance of image, v = -10 cm

We know that,1/v - 1/u = 1/f

So,  

1/-10 - 1/u = 1/-15

So, -1/10 + 1/15 = 1/u

(-3 + 2)/30 = 1/u

-1/30 = 1/u

Therefore u = -30 cm

The object is placed 30 cm away from the concave lens