Question

# The unit of power of a lens is:A. meterB. dyneC. dioptreD. newton

Hint: Define a lens. A lens will either converge or diverge the incoming light rays from each-other or from the principal axis. We can measure this amount of convergence or divergence of a lens. This gives the strength or power of a lens.

Power of a lens can be defined as the bending strength of a lens. The more the lens can bend the light rays passing through it, the more the power of the lens.
A convex lens will bend the light rays passing through the lens towards the principal axis i.e. convex lens converges the light rays. For a convex lens the power of the lens can be defined as the converging power of the lens. The more the lens can converge the light rays, the more is the power of the lens.
A concave lens will bend the light rays away from the principal axis i.e. the concave lens diverges the light rays. For a concave lens the power of the lens can be defined as the diverging power of the lens. The more the lens can diverge the light rays passing through the lens, the more is the power of a lens.
The power of a lens can be defined as the reciprocal of the focal length of the lens. Mathematically we can express this as,
$P=\dfrac{1}{f}$
Where, P is the power of the lens and f is the focal length of the lens in meters.
The unit of power of the lens is dioptres(D).

The correct option is (C).

Note:
If we have two thin lenses of power ${{P}_{1}}$ and ${{P}_{2}}$ close together, the total power of the lens system is given as,
$P={{P}_{1}}+{{P}_{2}}$
A convex lens has positive power because it converges the light rays passing though it towards each other. Again, a concave lens has negative power because it diverges the light rays passing through it away from each other.