Interesting Facts about the Power of a Lens

In ray optics, the idea of a lens’s power is one of the most intriguing. The following article provides an in-depth explanation of this chapter’s content so that students may better comprehend it.

Lenses in Ray Optics are powerful because of their ability to bend light. The more powerful a lens is, the more light that travels through it may be refracted. The converging and diverging abilities of a convex and concave lens are characterised by their respective powers.

You may use the following formula of power of a Lens Calculator in Ray Optics –

There are two types of lenses: Diopters, which measure power in diopters, and mm, which measure the focal length in metres. Keep in mind that the optical power of a converging lens is positive, whereas that of a diverging lens is negative.

This example gets 0.2 m when converting a lens’s focal length of 20 cm to metres. Taking the reciprocal of 0.2 gives us the power of this lens. As a result, the focal length’s power rating is 5D.

If you’ve read about the lens maker’s formula, you’ll know that what we’re calculating there is the lens’s power. A simple formula for determining how powerful a lens is is based on two surfaces’ radius and its refractive index.

An important use of exploiting the power of lenses is in ‘Optometry”. Deteriorating eyesight is the basis for prescribing corrective lenses (either a convex or concave lens). Your eye is essentially a lens, and you may have difficulty seeing clearly at times. Lenses that have adequate power may fix this problem.

To create a spherical lens, you need to join two spherical surfaces together. Spherical lenses may be divided into two categories:

  • The term “concave lens” refers to lenses created by glueing together two spheres and curving the resulting surfaces inward.
  • It is known as a convex lens if two spherical surfaces bulge outward when they are bound together.

Definition of the Lens Formula

Similarly, convex lenses are converging lenses, whereas concave lenses are referred to as diverging lenses because rays diverge when they fall on convex lenses. These lenses may produce either real or virtual images depending on where they are about the lens, and they can also vary in size.

You may use the lens formula to determine the image distance from an object’s distance and focal length. Formulas in optics are used to describe the connection between a lens’s focal length (f) and the distance between an image I and an object (o).

Convex and concave lenses may both use the same lens formula. The thickness of these lenses is quite low. In this equation, the focal length, the distance between the object and the image in the spherical mirror, and the object distance are all related. It’s stated as, ”


I = the distance between the lens and the picture.

o= distance between the lens and its subject

f = lens focal length

You may use the lens formula in any case where sign conventions are followed. This lens formula works for both concave and convex lenses. You may use image distance to determine whether or not an item appears on the other side of a lens.

For a diverging lens, a negative value in this equation indicates that it is a diverging lens instead of a converging lens. Real or virtual pictures may be measured using this equation.

Using the lens formula to calculate magnification:

Magnification is the ratio of the image’s height to the object’s height. In addition to the picture and object distances, it is also stated. In other words, it is the ratio of picture distance to object distance.


Where m stands for magnification.

The image’s height is referred to as hi.

h = object’s height

The magnificence of the lens

Convergence and divergence of incident light are used to calculate the lens’ power. The amount of convergence or divergence is determined by the lens’s focal length. Thus, the reciprocal of the focal length of the lens utilised is the power of the lens. It’s stated as, ”

F is the lens’s focal length in millimetres. Dioptre is the SI unit of power (D). Both the concave and convex lenses may have positive or negative power.

Dioptres are the standard unit of power for lenses.

A one-dioptre lens has a focal length of one metre, and D denotes the power of a lens. The doctor’s prescription for a person’s eyeglasses includes this dioptric number.

D = 1/Focal length in metre

= 100/Centimeter focal length

A convex lens has more power, whereas a concave lens has less.

As a result, a +2.5 dioptre lens with a focal length of 40 cm is considered convex. Concave lenses with focal lengths up to 20 cm have dioptre powers equal to 5 dioptres.

Problems to Solve:

  1. A lens is made by nearby two narrow lenses with powers +12 D and –8 D. This combo will have a what?


P1 is equal to +12 days, whereas P2 is equal to –8 days.

Combination (P) = P1 + P2.

This is the result of subtracting eight from 12

= 4

F = 1/P + 4/+4 = 1/f.t

0.25 micrometres

equates to 25 centimetres (Ans.)

  1. In this case, an optician recommends glasses to a patient with a combination of a 40-cm convex lens and a 25-cm concave lens for the patient. What kind of impact will spectacles have?


F1 = 40 cm

F2 = 0.25 m / 25 cm = 0.25 in

P1 = 1/F1 = 2.5 D

In a concave mirror, P2 = – 1/F2 = –1/0.25 = –40 D (Because P = –1/F)

Stimulating the senses

1+1=2, and so on.

In other words, 25 + (–40) =

= − 15 of a decimal degree (Ans.)


The focal length of a lens affects its ability to bend light rays. The more the bending of a beam of light, the narrower the focal length. A lens’ ability to bend light rays is directly related to the lens’ focal length.


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