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Chapter: 10th Science : Chapter 2 : Optics

Power of a Lens

When a ray of light falls on a lens, the ability to converge or diverge these light rays depends on the focal length of the lens.

POWER OF A LENS

When a ray of light falls on a lens, the ability to converge or diverge these light rays depends on the focal length of the lens. This ability of a lens to converge (convex lens) or diverge (concave lens) is called as its power. Hence, the power of a lens can be defined as the degree of convergence or divergence of light rays. Power of a lens is numerically defined as the reciprocal of its focal length.

P = 1/f                  …….  (2.6)


The SI unit of power of a lens is dioptre. It is represented by the symbol D. If focal length is expressed in ‘m’, then the power of lens is expressed in ‘D’. Thus 1D is the power of a lens, whose focal length is 1metre. 1D = 1m-1.

By convention, the power of a convex lens is taken as positive whereas the power of a concave lens is taken, as negative.


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10th Science : Chapter 2 : Optics : Power of a Lens |


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