aberration



aberration defined in 1939 year

aberration - Aberration (Lat. ab, from err are, to wander);
aberration - Optical phenomenon of two kinds, spherical and chromatic. Spherical aberration may be defined as follows: When a luminous object is placed opposite a small, spherical, concave mirror, the rays of light falling on the mirror are all reflected back so as to pass through one point on the axis of the mirror. But if the mirror is large the reflected rays no longer pass through a single point, those which arrive from points of the mirror farther from its axis cutting the axis nearer the surface of the mirror and farther away from the luminous source of light.

It is found that all the reflected rays are tangential to a curve which forms two cusps inside the curve of the mirror. This caustic curve, as it is called, can be clearly seen when a bright light shines on the inside of a cup nearly filled with milk, the surface of the milk acting as a screen to intercept the reflected rays from the cup's inner surface. Spherical aberration of light images from the surface of a concave mirror is the simplest theoretical instance of the aberration which takes place when light passes through lenses compounded of concave and convex surfaces.

Chromatic aberration in lenses is an illustration of a different principle. White light in passing through a prism, that is to say in passing through the medium of the atmosphere into the medium of glass, is bent or deviated at the surface of the glass. It is also dispersed on its emergence from the prism into the band of rainbow colours which are the constituents of white light. When white light, i.e. light of mixed colours and therefore of mixed wave lengths, passes through lenses these laws come into operation. But the amount of dispersion sustained by the different colour constituents of the white light is not the same, and it differs with the character of. the glass. Consequently, when light, by which is meant mixed light or light of differing colours and wave lengths, passes through a system of lenses, the image of the source of light is projected in different colours and different places and in the different aberrations. If all these images are intercepted by a plane surface, such as the retina of the eye or the focusing screen of a camera, they arrive in confusion This is chromatic aberration. One effect is the image seen on a dark background which has a rainbow-coloured fringe.

In astronomy, aberration is the displacement of a star, arising from the progressive motion of light combined with the orbital movement of the earth. It provided the basis for determining the speed of light. See Achromatic Lens; Astronomy; Light.

near aberration in Knolik


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