What is meant by power of a lens? Define its S.I. unit.
You have two lenses A and B of focal lengths +10 cm and -10 cm respectively, State the nature and power of each lens. Which of the two lenses will form a virtual and magnified image of an object placed 8 cm from the lens? Draw a ray diagram to justify your answer.

Question

What is meant by power of a lens? Define its S.I. unit.
You have two lenses A and B of focal lengths +10 cm and -10 cm respectively, State the nature and power of each lens. Which of the two lenses will form a virtual and magnified image of an object placed 8 cm from the lens? Draw a ray diagram to justify your answer.

Philoid · Accepted Answer

Power of the accommodation of an eye is defined as the ability of an eye to change its focal length so as to focus the distant objects as well as nearby objects on the retina. The S.I. unit of power of lens is Diopter.

Given: focal length f₁ = +10 cm

In meters f₁ = +0.1 m

P = 1/f = 1/0.1

P = +10 D

There is positive sign on power, so lens A having focal length +10 cm is convex lens

focal length f₂ = -10 cm

In metres f₁ = -0.1 m

P = 1/f = 1/-0.1

P = -10 D

There is negative sign on power, so lens A having focal length -10 cm is concave lens

So, when the object is placed at 8 cm between optical center and focus on a convex lens, then image formed will be virtual and magnified. The ray diagram will be as follows: