Question

What is meant by power of a lens ? What does its sign (+ve or —ve) indicate ? State its S.I. unit. How is this unit related to focal length of a lens?

Answer 1

~The ability of converging or diverging of the light rays passing through the lens is known as power of the lens.
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~ The +ve sign of the power indicates that the lens is a converging lens or convex lens, whereas the negative sign of the power indicates that the lens is concave or diverging lens.
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~The S.I unit of the power is Dioptre or 1/metre.
Power = 1/focal length (in m)

Answer 2

The power of a lens is the measure of its ability to converge or diverge a parallel beam of light on refraction through it .Negative sign for Concave lens and Positive sign for Convex lens. The S.I.unit of power is dioptre (D).

    f = 1m

​      P = 1/1m  = 1m-1 = 1dioptre (D)

      P=1/f (in  m)  =100/ f(in  cm)