Question

What is meant by power of a lens? Write it's SI unit . A student uses a lens of focal length 40 cm and another of -20cm . Write the nature and power of each lens

Answer 1

POWER OF A LENS :
The power of a lens is reciprocal of the focal length. It is represented by the letter P.

The SI unit of power of a lens  is dioptre. It is denoted by the letter D.

Power of lens(P) = 1/focal length of the lens (in m)
P = 1/f (in m)

•The power of lens is inversely proportional to its focal length.
A lens of short focal length has more power where as a lens of a long focal length has less power.
•A convex lens has a positive focal length and a concave lens has negative focal length.
• Dioptremetre is an instrument for measuring the power of a lens.

In case 1:
Focal length (f) = 40 cm = 40/100= ⅖= 0.4 m       [1 cm =1/100m]
P = 1/f
P = 1/0.4
P = 10/4 = + 2.5 D 
P =  + 2.5 D

In case 2:
Focal length (f) = - 20 cm = - 20/100= -⅕ = - 0.2 m           [1 cm =1/100m]
P = 1/f
P = 1/ - 0.2P = - 10/2 = - 5 D
P = - 5 D

Lens of power + 2.5 D(40 cm focal length) is converging (convex) lens of power - 5 D(-20 cm focal length) is diverging (concave).

HOPE THIS WILL HELP YOU...

Answer 2

Answer:

ok the answer is write thankyou teacher