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What is the SI unit of the power of a lens?
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Answer: Option A โ Dioptre
Detailed Solution
The power of a lens is a measure of its ability to converge or diverge light rays passing through it. It is defined as the reciprocal of its focal length, when the focal length is measured in metres:
P = 1/f (with f in metres)
The SI unit of power of a lens is the dioptre, represented by the symbol D. One dioptre is the power of a lens whose focal length is one metre. So a lens with a focal length of 0.5 m (50 cm) has a power of 2 D, and one with a focal length of 25 cm has a power of 4 D. A lens with a shorter focal length bends light more strongly and has a greater power.
By the sign convention:
- A convex (converging) lens has a positive focal length, so its power is positive (for example, +2.0 D).
- A concave (diverging) lens has a negative focal length, so its power is negative (for example, โ1.5 D).
- When two or more thin lenses are placed in contact, the total power is the sum of their individual powers: P = P1 + P2 + P3 + ...
- Opticians and eye doctors write the power of corrective lenses in dioptres on spectacle prescriptions:
- Myopia (short-sightedness), in which a person cannot see distant objects clearly, is corrected with a concave lens (negative power).
- Hypermetropia (long-sightedness), in which a person cannot see nearby objects clearly, is corrected with a convex lens (positive power).
- Presbyopia (age-related difficulty in focusing on nearby objects) is often corrected with bifocal lenses.
- Astigmatism is corrected with cylindrical lenses.
Why not the other options?
- โWatt is the SI unit of power in the sense of energy per second (mechanical or electrical power).
- โLumen is the unit of luminous flux.
- โCandela is the SI unit of luminous intensity.
Quick tip
Remember: Power of lens P = 1/f (metres), unit dioptre (symbol D); convex = positive power; concave = negative; myopia = concave lens; hypermetropia = convex lens.
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Light & Optics โ Preparation
This question tests your understanding of Light & Optics, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers the nature of light, reflection and refraction, mirrors and lenses, total internal reflection, dispersion and scattering of light, the human eye and its defects, optical instruments, and the colours of objects. Exams test the phenomena behind everyday sights: the sky looks blue because of the scattering of light, rainbows form through dispersion and internal reflection, diamonds sparkle because of total internal reflection, and mirages are caused by refraction. Know which lens corrects which defect (concave lenses for myopia and convex lenses for hypermetropia). Practise the light and optics questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.