Question

What do you mean by power of lens? What is the S.I. unit of the power of lens? Calculate the power of a concave lens of focal length \[25\] cm.

Answer 1

Hint:Concave lens is a diverging lens when a ray of light falls on it they diverge them or spread out them. Their centers are thinner than the edges. When light coming from infinity strikes the lens they seem to be converged at focus. The distance between focus and optical center of lens is the focal length. Concave mirror has a negative focal length.

Complete step by step answer:
Power of a lens is numerically equal to the reciprocal of its focal length in meters. It is defined as the ability of a lens to converge or diverge a light ray when it refracts through it.
\[P = \dfrac{1}{f}\]
S.I. Unit of power is Diopter (D), \[1\] diopter is equal to power of lens when focal length of lens is taken \[1\] m.
As given in question:
$f = - 25\,cm$
\[\Rightarrow P = \dfrac{1}{{25 \times {{10}^{ - 2}}}}\]
\[\Rightarrow P = \dfrac{{100}}{{25}}\]
\[\therefore P = - 4D\]

Note:When two or more lenses are kept close, the power of combination is equal to addition of all individual powers of lens. Plane mirrors have zero power. Convex lenses have positive power as they converge to light beams. More is the power of lens implies more is its ability to produce deviation of light.