achromatic lens

achromatic lens defined in 1939 year

achromatic lens - Achromatic Lens (Gr. achromatos, colourless);
achromatic lens - Optical instrument for correcting the chromatic aberration of white light. When a narrow pencil of white light, such as sunlight, is allowed to pass from one medium to another, as from air into glass, the pencil of rays is not merely bent, but is split up into light of several colours. This phenomenon is referred to as dispersion.

But the dispersive powers of different substances are not the same. We can obtain two prisms constructed of such different materials that, while the angle through which they will bend the average or mean ray of a pencil of light is the same, the amount of dispersion is different; or, on the other hand, when the deviation or bending of the mean ray is different, the amount of dispersion produced is the same. The dispersion produced by flint glass is greater than that produced by crown glass in the ratio of 61 to 43. If two prisms, one of crown glass and one of flint glass, and of suitable angles, are placed with their refracting edges turned in opposite directions—the second being upside down in relation to the first—a ray of white light passing through the first (crown glass) will be bent downwards towards the base, but the red constituent of the ray will be less bent than the violet owing to dispersion.

When these rays, however, having passed through the crown glass prism, set out on their journey through the reversed flint glass prism, the bending or deviation will be in the opposite direction; but the red ray will be less bent upwards than the violet. The difference between the deviations of the red and violet rays being, however, the same in the two prisms, the rays when they leave the second, or flint glass, prism will be parallel. Nevertheless, there will be a general deviation of the rays on the whole because the average or mean deviation in the crown prism is greater than that in the flint. Hence, by combining two prisms, one of crown glass and one of flint glass, the ratio between the refracting angles of the prism having been suitably chosen, we obtain a compound prism which deviates but does not disperse light.

This general principle is applied to construct achromatic lenses, which correct the chromatic aberration of white light when it passes through a lens, and when the curvature of the lens disperses the different coloured rays to different foci. Suppose, for example, a parallel beam of white light impinges on a convex lens, then where the rays enter and leave the lens the violet rays will be more deviated towards the axis of the lens than the red rays, and thus will come to a focus nearer the lens than the focus of the red rays. With a concave lens the conditions are reversed. If a concave lens and a convex lens of the same material and of equal focal length were placed together, the two dispersions would counteract one another: though in this case there would be no deviation.

But by making the convex lens of crown glass and the concave lens of flint glass, we are able, as in the case of the prisms, to obtain equal and opposite dispersion and still have deviation in the direction of that produced by the crown glass. In effect the combination will be a convex lens with the chromatic aberration eliminated—in other words, an achromatic lens. By the use of two lenses it is possible to make a lens which shall be achromatic as far as the light of two colours is concerned. The combination will not be achromatic for other colours. If instead of two lenses we use three, of different dispersive materials and powers, the combination can be made achromatic for three colours, and so on. The colours for which the lens system is rendered achromatic vary with the uses to which it is put. Thus, in a telescope, achromatism is secured for rays which immediately affect the eye; in photography the lens system must be achromatised principally for the violet rays, and for rays beyond the violet which are called actinic. See Aberration; Light; Spectroscopy.

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