Question

What is meant by powre of lens. A student uses a lens of focal length of 40 cm a'd another 20cm . write the nature and power of each lens . according 3 marks​

Answer 1

Explanation:

Power of lens is known as the inverse of the focal length of the lens measured in meter(m).

S.I unit of power = Dioptre (D)

Power of lense = 1/ focal length in meter.

The focal length of 1st lense f1 = 40 cm = 0.4 m

The lense used is a convex lens because the focal length is a positive value.

Power of lense = 1/ focal length

P1 = 1/f1

P1 = 1/0.4

P1 = 2.5 D

The focal length of 2nd lense f2 = -20 cm = – 0.2 m

The lense used is a concave lens because the focal length is a negative value.

P2 = 1/f2

P2 = 1/(-0.2)

P2 = -5 D