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Thread: Achromats

  1. #1

    Achromats

    What makes a low-refractive-index convex lens placed before a high-refractive-index concave lens an achromatic doublet? I mean, are there any specific ratios/relations between the radii of curvatures of the surfaces, refractive indices of the lenses etc.?

  2. #2

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    Urban Domeij

    Re: Achromats

    The second question has to be answered with yes. The first one is trickier.

    The net result of the two lenses should be that chromatic dispersion of one cancels out chromatic dispersion of the other, while the sum of the two is still a positive lens, in case that is what is designed, or negative if that is the goal. So to make a high diopter positive system, a highly refractive positive lens with low dispersion is combined with a lower refraction negative lens with higher dispersion, but that dispersions of the two lenses amounts to the same, so that the negative dispersion cancels out the positive.

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