Define power of a convex lens. What are its units?

Hint:Power of a lens is the quantity which defines its light gathering capacity or we can say how much it diverges the light, by what angle. Convex lens power is positive while for the concave lens it is negative. Power means the ability of the lens to bend to make a sharp image on the retina.

Complete answer:
The focal length of a convex lens is always positive; hence it is a converging lens that is convex. Now we know the relationship between focal length and power is given by,$P =\dfrac{1}{f}$. Here focal length is always taken in metres. Power of a lens is defined as the reciprocal of the focal length in (m). It is measured in diaptore.

Additional Information:
More is the power lesser is the focal length and vice versa. Also, one or more than one lens can be combined and then its power of combination can be determined. Power of combination of the lens is simply the addition of individual lenses provided they are placed close to one another and there is no distance between them.

Note:While calculating power always take the focal length in metres. For convex lenses take the focal length as positive and for concave lenses take the focal length as negative. Convex lens converges parallel rays of light while a concave lens diverges parallel rays of light. Convex lens is thin from the edges about thick at the middle. Always keeps focal length in metres, while finding the power otherwise the power will not come in units of diaptore.