alleryPower of a corrective lens is expressed in dioptres. One dioptre equals:
A1 per metre (m^-1)
B1 metre
C1 centimetre
D1 newton per metre
Answer & Solution
Correct answer: A. 1 per metre (m^-1)
1. Power of a lens is the reciprocal of its focal length in metres.
2. P = 1 / f, where f is in metres.
3. The SI unit of P is m^-1, which is called the dioptre (D).
4. So one dioptre equals 1 per metre.
5. A lens of f = 50 cm has power = 1 / 0.5 = 2 D.
_Source: syllabus Science Ch 11 'Human Eye and the Colourful World'_
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