Question

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?

Answer 1

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) = \(\frac{1}{f(in\,meters)}\)

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.