Example 1 · level 1
A convex lens has a focal length of +200 cm. Find its power in dioptres, with sign.
Show solution
- f = 200 cm = 2 m.
- P = 1/f = +0.5 D.
Answer: 0.5 D
CLASS 10 SCIENCE · LIGHT: REFRACTION
Power of a lens (dioptre)
P = 1/f with f in metres (P = 100/f with f in cm). Unit: dioptre (D). Convex lens: positive power; concave lens: negative. Powers of lenses in contact add.
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Worked examples
Example 2 · level 2
Two thin lenses of focal lengths +25 cm and +100 cm are kept in contact. Find the power of the combination in dioptres.
Show solution
- P₁ = 100/25 = +4 D.
- P₂ = 100/100 = +1 D.
- P = +5 D.
Answer: 5 D
Example 3 · level 3
An optician prescribes a lens of power +2.5 D to Tenzin. Find its focal length in cm, with sign.
Show solution
- f = 1/(+2.5) m = 0.4 m = +40 cm.
- Positive: a convex (converging) lens, used for hypermetropia.
Answer: 40 cm