Dioptre is the SI unit of :

size of the lens.

curvature of the lens.

power of the lens.

magnification produced by a lens.

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UPSC CISF-AC-EXE – 2022

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Dioptre (symbol D) is the unit of measurement for the optical power of a lens or curved mirror.
Optical power (P) of a lens is defined as the reciprocal of its focal length (f) when the focal length is measured in metres.
P = 1 / f (where f is in metres)
If the focal length f is in metres, the unit of power P is m⁻¹, which is defined as the dioptre (D).
For example, a lens with a focal length of +0.5 metres has a power of P = 1 / 0.5 m = +2 D.
A lens with a focal length of -0.25 metres has a power of P = 1 / -0.25 m = -4 D.

– Dioptre is the unit used to measure the optical power of lenses.
– Optical power is the reciprocal of the focal length measured in metres.

Size of the lens is measured in units of length (e.g., diameter in cm). Curvature of the lens is related to the radii of curvature of its surfaces, usually measured in reciprocal metres (m⁻¹), and contributes to the power of the lens along with the refractive index of the material. Magnification is a ratio of image size to object size, which is a dimensionless quantity or expressed as ‘x’. Dioptre specifically quantifies the power of the lens to converge or diverge light.

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