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27 Dec 2011 10:04:07 IST

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lens makers formula

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lens makers formula

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Viraat Gupta

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Joined: 21 Jun 2011

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28 Dec 2011 22:49:54 IST

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If d is small compared to R₁ and R₂, then the thin lens approximation can be made. For a lens in air, f is then given by

$\frac{1}{f} \approx \left(n-1\right)\left[ \frac{1}{R_1} - \frac{1}{R_2} \right].$

The focal length f is positive for converging lenses, and negative for diverging lenses. The reciprocal of the focal length, 1/f, is the optical power of the lens. If the focal length is in metres, this gives the optical power in dioptres (inverse metres).

Lenses have the same focal length when light travels from the back to the front as when light goes from the front to the back, although other properties of the lens, such as the aberrations are not necessarily the same in both directions.

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