Question

Size of the image of an object by a concave lens of focal length 20cm is observed to be reduced to 1/3 of its size.
FIND THE DISTANCE OF THE OBJECT FROM THE LENS.

Answer 1

Object distance -40 cm

Answer 2

Answer:

The focal length of concave lens, f = -20cm.

According to the question, size of image = 1/3 × size of object.

So, magnification, m = 1/3 ( as lens is concave so, magnification always be positive). We know that the magnification of lens, m = v/u or, 1/3 = v/u => v = u/3 ......(1).

Using lens maker formula, 1/v - 1/u = 1/f or, 1/(u/3) - 1/u = 1/-20 from equation (1), or, 3/u - 1/u = 1/-20 or, 2/u = 1/-20 or, u = -40cm.

Therefore, the distance of object from the lens is 40cm.