Spectacles differ based on the power of the lens used in it.

Before beginning with the power of a lens, let’s understand some basics

A convex lens can converge a beam of parallel rays to a point on the other side of the lens.

This point is called a focal point and its distance from the center of the lens is called focal length.

Similarly, a concave lens diverges a straight beam of light coming from the source.

The point where the ray of light seems to refract and diverge is called principal focus and its distance from the centre of the lens is called focal length.

Now let us learn about Power of Lens

Power of Lens is defined as the converging or diverging capacity of the lens.

It can also be stated as the reciprocal of focal length.

Power of the lens is given by, $P=f1 $
Where,$f=$ focal length

S.I. unit of power is Diopters (D) or $Meter_{−1}(m_{−1})$

The power of the lens is positive for the convex or converging lens while it is negative for a concave or diverging lens.

Let us now understand the power of the human eye

The total optical power of the human eye is 60 Diopters.

So, when it is reduced, a lens with a positive power or convex lens is used for correction.

And a lens with negative power or a concave lens is used, when the power of eyes is increased.

Now we’ll study a combination of lens

When more than one lens is used, the power of the lens is added.

Consider two lenses with focal lengths $f_{1}$ and $f_{2}$ are arranged as above.

The power of the system of the lens is given as,$P=P_{1}+P_{2}$
Where, $P_{1}=$ power of lens $l_{1}$$P_{2}=$ power of lens $l_{2}$

Therefore,$P=f_{1}1 +f_{2}1 $
Where $f_{1}$ and $f_{2}$ are focal lengths of lens $l1$ and $l2$ respectively.

Revision

The power of the lens is defined as the reciprocal of the focal length of a lens.

Therefore, $P=f1 $
Where f is the focal length

SI unit of power of the lens is Diopter or $meter_{−1}.$