What is meant by power of a lens? Name and define its S.I. unit.

One student uses a lens of focal length +50 cm and another of –50cm. State the nature and find the power of each lens. Which of the two lenses will always give a virtual, erect and diminished image irrespective of the position of the object?

Power of lens is defined as the efficiency with which a lens can converge or diverge the light ray.

The reciprocal of focal length of the lens is called the power of the lens (P).


P (in dioptre) =


The S.I unit of power of lens is dioptre denoted by D.


f1 = 50cm = 0.5m (1m = 100cm)
⇒ P1 = 1/f1
P1 = 1/0.5
P1 = 2D
Since Power is positive therefore the lens with focal length 50cm is convex lens.
f2 = -50cm = -0.5m (1m = 100cm)
⇒ P2 = 1/f1
P2 = 1/-0.5
P2 = -2D
Since Power is negative therefore the lens with focal length -50cm is concave lens.
The image formed by the concave lens is virtual, erect and diminished irrespective of the position of the object.


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