The power of a lens is measured in dioptres and is equal to
Correct answer: B. the reciprocal of the focal length in metres
- A. the focal length in metres
- B. the reciprocal of the focal length in metres
- C. the focal length in centimetres
- D. the magnification produced
Explanation
A lens of focal length 0.5 m has a power of 2 dioptres, and a shorter focal length means a more strongly converging lens. Diverging lenses have negative focal lengths and therefore negative powers, which is how spectacle prescriptions for short sight are written. Powers of lenses in contact simply add, which is why the unit is convenient.
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About Optics
Light is studied through reflection, refraction, dispersion, interference, diffraction and polarization, linking ray behavior with its wave nature. Questions may involve mirrors, lenses, optical instruments, refractive index and image formation, while geometrical optics deals with rays and wave optics explains effects such as interference and diffraction.
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