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