Question

what is meant by power of a lens write its SI unit a student uses a lens of focal length 40 cm and other of -20 CM write the nature and power of each lens

Answer 1

It's the ability of a lens to converge or diverge a ray of light passing through it. It's SI unit is diopter(D)
Power(P) = 1/focal length(in m)
1st lens:
P = 1/0.4m
=10/4
=2.5 D
Since it is +ve its a convex lens.
2nd lens:
P = 1/-0.2m
= 10/-2
= - 5 D
Since it is - ve its a concave lens.

Answer 2

It is a ability of lens which diverge or converge the ray of light passing through it is called power of the lens.

It's SI unit is "Diopter " (D).

In Case I

P = 1

     f (in m )

To convert focal cm to m we divide it by 100

For ex :40/100

= 0.4m

P=1/f

=1/0.4

= P = 10/4 = 2.5 D

It is a convex lens because the power of the lens is positive

In Case Il

P=1/f

=1/-0.2

= P = 10/-2 = -5 D

It is a concave lens because the power of the lens is negative.