The power of a lens is defined as the ability of the lens to converge or diverge a beam of light falling on it. A lens is said to be of greater power if it diverges or converges a beam of light more strongly by focusing them closer to the optical centre. In other words, a lens of small focal length is said to have greater power. On the other hand, a lens is said to be of lesser power if it diverges or converges a beam of light to less extent. In other words, a lens of large focal length is said to be of lesser power.
Thus, the power of a lens is the reciprocal of focal length of the lens, i.e.,
Power = 
or
Power = 
where, p = power of the lens
f = focal length of the lens in meters
The power of a convex lens is positive as a convex lens has a positive focal length, while the power of a concave lens is negative as concave lens has a negative focal length.
S.I. unit of Power of Lens
The S.I. unit of power of a lens is ‘dioptre’. It is denoted by the symbol D. One dioptre is the power of a lens whose focal length is 1 meter. So, when f = 1 meter, then
Power =
or
Power = 
= 1 dioptre
Test Your Understanding and Answer These Questions:
- What is power of a lens? Give S.I. unit of power of a lens.
- Define 1 dioptre.
- Find the power of a concave lens of focal length 4 m.
- Find the focal length of a concave lens of power – 4 D.
- A convex lens forms a real and inverted image of a needle at a distance of 50 cm from the lens. Where is the needle placed in front of the convex lens, so that this image is of the same size as the object? Also, find the power of the lens.