Power Of Lens
Power of a lens is the measure of how strongly it converges or diverges light.
In one line: Shorter focal length means stronger power.
Practice This ConceptUnderstand it
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.
Example
Formula and rule
Power of lens P = 1/f, where f is in metres.
Unit: dioptre (D).
See it worked out
Follow the example line by line, then use the same method on new questions.
Worked example
- 1For f = 0.5 m, P = 1/0.5 = +2 D for a convex lens.
- 2If the lens were concave, the sign would be negative.
Method you can reuse
- 1Convert focal length to metres.
- 2Use P = 1/f.
- 3Assign sign according to lens type.
- 4State the unit dioptre.
Sketch it in your answer
Show a convex or concave lens, focal points, and the direction in which the lens brings rays together or spreads them apart.
Check yourself
Answer the question in your head before opening the answer.
Try it yourself
What is the power of a convex lens with focal length 50 cm?
Think first, then show answerHide answer
Common wrong answer
Leaving focal length in centimetres and writing 1/50 as the answer. The unit would then be incorrect.
Don't mix this up
Exam tip
Write it in the exam
Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.
How CBSE asks it
- 1
Enough for 1 mark
Power of a lens is the measure of how strongly it converges or diverges light.
- 2
Add for 2 marks
Example: A lens with focal length 0.5 m has power +2 D.
- 3
Add for 3 marks
For f = 0.5 m, P = 1/0.5 = +2 D for a convex lens.
Practice this concept with focused MCQs
Open the concept quiz intro first, review the test details, and then start a focused MCQ set from this concept only. Instant score and answer review are live now.
Help improve this page
Found something confusing, incorrect, or missing?