Question

Convex and concave lenses are kept close to each other. Magnitude of their powers are in ratio 3:5. If effective focal length is 60 cm and if it acts like a concave lens, find the focal lengths of individual lenses.

Answer 1

Given:

• Magnitude of their powers are in ratio 3:5

• Effective focal length is 60 cm.

To find:

The focal lengths of the individual lenses.

Solution:

1) The power of the lenses must be 3x and -5x so the effective power of the combination will be -2x.

2) The effective focal length is 60 cm and power = 1/ focal length

• -2x = -1/60

• x = 1/120

• 3x = 1/40

• 5x = 1/24

3) The focal length of the convex lens is 40 cm and concave lens is -24 cm.

The focal lengths of the individual lenses are +40 cm and -24 cm.