The power of a lens is a measure of the degree of convergence or divergence of light rays falling on it.
The power of a lens is defined as the reciprocal of its focal length in meters.
Power of lens=1focal length of lens(m)
P=1f(meters)
P=100 f(cm)
Note:-
- Unit of power of lens in Dioptre(D)
- The power of concave lens is negative(-ve)
- The power of convex lens is positive(+ve)
- A thick lens has short focal length but more power. A thin lens has large focal length but less power.
Power of a combination of lenses:
If a number of lenses are placed in close contact, then the power of the combination of lenses is equal to the algebraic sum of the powers of individual lenses.
P=p1+p2
Example: Suppose convex lens has power +4D and concave lens of power -10D,So resultant power should be (4+(-10)=-6D) 6D. So this combination look like.
P= +4+(-10)
= +4-10
= -6 D
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| Power of a combination of lens |