Question

An object is placed at a distance of 15 cm from a concave lens of focal length 30 cm. List four characteristics (nature, position, etc.) of the image formed by the lens.

Answer 1

Given:

Distance of object from lens, u = 15 cm

Focal length of concave lens, f = −30 cm (negative because concave lens)

Use the lens formula

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

`1/-30 = 1/v - 1/15`

v = 30 cm

But since this is a concave lens, the image forms on the same side as the object, so

v = −10 cm

Characteristics of the Image

For a concave lens, the image formed is always

1. Virtual: because rays do not actually meet.

2. Erect (upright): same orientation as the object.

3. Diminished: smaller in size than the object.

4. Position: between the optical centre and focus (F) on the same side as the object.