Alfred Morgan Jr., age 16, of Orlando, Fla., for his question:
CAN YOU EXPLAIN ABERRATION?
Aberration is a term used in physical science. The word comes from Latin and means “to wander from a given path.” There are three important kinds of aberration in physical science: special aberration, chromatic aberration and astronomy aberration.
Spherical aberration is the failure of light rays to meet at a point when they are reflected by a concave mirror or bent when passing through a convex lens.
When parallel light rays strike a spherical concave mirror, the light rays that strike the center of the mirror focus at a point farther from the mirror than those that are reflected from points near the outer edge of the mirror. Instead of meeting at a common focal point, these light rays meet along a focal curve called the caustic curve.
If a glass is almost filled with milk, and the light falls on the wall of the glass from an angle, a caustic curve will be seen on the surface of the milk.
There is a similar curve formed by spherical aberration when the light passes through a spherical convex lens. In order to lessen this aberration, it is possible to cut off some of the light rays that pass through the lens near the edge. in cameras, this is done by changable openings called stops. The smaller the opening through which light passes on its way to the lens, the sharper is the image formed on the film.
Spherical aberration can also be controlled in a lens by changing the curvature of the lens. Such a lens is called an aplanatic lens. A lens for a large telescope, for example, sometimes has its edges flattened by polishing. A parabolic mirror does not have spherical aberration.
Chromatic aberration is the failure of the different colors contained in white light to meet in a common point, called a focal point, after they pass through a convex lens.
In astronomy, aberration causes heavenly bodies to appear to be at positions that are different from their actual positions in space.
More about chromatic aberration: when light passes through a double convex lens, the lens serves as a double prism. Red light is bent the least when it strikes the lens and violet is bent the most.
Chromatic aberration is found in all single lenses. But by combining two or more lenses made of different kinds of glass, the various colors in light can be made to meet at practically a single focal point. A lens made of such a combination is called an achromatic lens.
If the earth were at rest, a star could be observed by pointing a telescope directly at the star. But because the earth is moving rapidly, the eyepiece of the telescope is traveling while the light is passing through the telescope slightly, so that the eyepiece will be in the proper position to receive the light ray when it arrives. The angle between the actual and the apparent position of the star is the angle of aberration.