Aberration, pronounced ab uh RAY shuhn, in optics, is the failure of light rays to focus properly after they pass through a lens or reflect from a mirror. Proper focus occurs when the light rays cross one another at a single point. There are two types of optical aberration, spherical aberration and chromatic aberration.

Lenses or mirrors that are sections of spheres produce spherical aberration. If a beam of parallel rays reflects from a concave mirror, the rays that reflect from the center of the mirror cross one another at a single point. The rays that reflect far from the center cross at points closer to the mirror surface. The imaginary line connecting these points of focus is called a caustic.

A caustic appears as a bright line if it shines on a surface. For example, when sunlight shines through the open top of a glass of milk and onto the curved interior of the glass, the interior acts as a mirror. As a result, the light reflects onto the milk in a caustic curve. Without aberration, a bright spot would appear on the milk.

Convex lenses also produce spherical aberration. The light rays that pass through the middle of the lens focus farther from the lens than do the rays that pass through the lens at the edges. If the lens is in a camera, the image on the film is blurry. To sharpen the image, a camera has a small opening called a stop. The stop allows only the rays passing through the center of the lens to reach the film. Thus, the rays focus at one spot on the film, and the picture is clear.

Chromatic aberration is the failure of different colored light rays to focus after passing through a lens. When white light, which consists of all colors, passes through a lens, the lens bends the rays. The rays then cross one another on the other side. The violet rays bend more than the other colors and focus close to the lens. The red rays bend the least and focus farther from the lens. Rays of the other colors focus at points between these two points.

A third type of aberration, called astronomical aberration, has nothing to do with the failure of light rays to focus. Astronomical aberration results from a combination of the earth's motion and the time it takes the light from a star to travel through a telescope. If the earth were stationary, an observer could point a telescope directly at a given star. However, the earth and the telescope move while the light is passing from the upper end of the telescope to the eyepiece. The observer must therefore tilt the telescope slightly so that the rays pass through the eyepiece instead of striking the interior of the telescope. As a result, the star appears in a position other than its actual position. The angle of difference between these two positions is called the angle of aberration.

Contributor: Jearl Walker, Ph.D., Prof. of Physics, Cleveland State Univ.