Physics
Study Material

# Power of Lens for Single and Combination of Lens

Power of lens mathematically is the inverse of focal length. One dioptre is the measure of convergence or divergence by a lens of focal length 1 meter.

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Posted by Mahak Jain, 27/9/2021 Hesap Oluştur

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Power of lens is simply the ability to bend light. More is the power, more is the bending of light and object appears closer. This concept is discussed in detail below. In other words, power of a lens is a measure of the convergence or divergence, which a lens introduces in the light falling on it. For concave lens it measure of divergence and for convex lens it is measure of convergence.

## Define Power of Lens

Power of lens is measure of the convergence or divergence, which a lens introduces in the light falling on it. Thus, for lens with larger focal length, power is less and vice versa. So, we can say that power is inversely proportional to focal length of the lens.

⇒ P ∝ 1/f

Here, constant of proportionality is 1 (by experiments).

Therefore, P = 1/f

This is formula for power of lens.

Hence, mathematically power of lens is inverse of focal length of the lens with sign convention.

The power of a lens of focal length of 1 metre is one dioptre.

## SI Unit of Power of Lens

SI unit of power of lens is Dioptre. It is denoted of D.

Dioptre = 1/meter

Or, D = m-1.

## What is Meant By Power of Lens?

If focal length of a concave lens is 5cm, then power is -20 D.

Focal length of concave lens is negative. So, f = -5cm = -0.05m.

⇒ P = 1/(-0.05)

Thus, P = -20 D.

If focal length of a convex lens is 5cm, then power is 20 D.

Focal length of concave lens is negative. So, f = 5cm = 0.05m.

⇒ P = 1/(0.05)

Thus, P = 20 D.