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Power Of Lens

Power of a lens is the measure of how strongly it converges or diverges light.

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Student-friendly explanation

Power is the reciprocal of focal length in metres: P = 1/f. A convex lens has positive power because it converges light, while a concave lens has negative power because it diverges light. The unit is dioptre.

How to write this in exams

  1. 1

    Start with the exact idea

    Power of a lens is the measure of how strongly it converges or diverges light.

  2. 2

    Then show how to use it

    1. Convert focal length to metres. 2. Use P = 1/f. 3. Assign sign according to lens type. 4. State the unit dioptre.

  3. 3

    Add one concrete example

    A lens with focal length 0.5 m has power +2 D.

  4. 4

    Avoid this incomplete answer

    Leaving focal length in centimetres and writing 1/50 as the answer. The unit would then be incorrect.

Definition

Power of a lens is the measure of how strongly it converges or diverges light.

Example

A lens with focal length 0.5 m has power +2 D.

Rule to remember

Power of lens P = 1/f, where f is in metres. Unit: dioptre (D).

Memory hook

Shorter focal length means stronger power.

Examples and method

Worked example

For f = 0.5 m, P = 1/0.5 = +2 D for a convex lens. If the lens were concave, the sign would be negative.

Method to apply

1. Convert focal length to metres. 2. Use P = 1/f. 3. Assign sign according to lens type. 4. State the unit dioptre.

Diagram support

Show a convex or concave lens, focal points, and the direction in which the lens brings rays together or spreads them apart.

How CBSE asks it

Questions may ask you to calculate power from focal length, compare lens strength, or identify lens type from the sign.

Avoid common mistakes

Common confusion

Students forget to convert centimetres into metres before finding power.

Common wrong answer

Leaving focal length in centimetres and writing 1/50 as the answer. The unit would then be incorrect.

Exam tip

Always write the sign of power with the lens type: convex positive, concave negative.

Quick check

What is the power of a convex lens with focal length 50 cm?

The power is +2 D because 50 cm = 0.5 m, and power equals 1 divided by focal length in metres.

Answer writing and exam use

1-mark answer

Power of a lens is the measure of how strongly it converges or diverges light.

2-mark answer

Power of a lens is the measure of how strongly it converges or diverges light. Power of lens P = 1/f, where f is in metres. Unit: dioptre (D). A lens with focal length 0.5 m has power +2 D.

3-mark answer

Power is the reciprocal of focal length in metres: P = 1/f. A convex lens has positive power because it converges light, while a concave lens has negative power because it diverges light. The unit is dioptre. Power of lens P = 1/f, where f is in metres. Unit: dioptre (D). For f = 0.5 m, P = 1/0.5 = +2 D for a convex lens. If the lens were concave, the sign would be negative. Questions may ask you to calculate power from focal length, compare lens strength, or identify lens type from the sign. Leaving focal length in centimetres and writing 1/50 as the answer. The unit would then be incorrect.
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