Optics. Aberration and its types. Absorption and adsorption

Aberration, in optics, is the failure of a lens or mirror to produce an image that is sharply focused and has the same proportions as the object being viewed. There are three main kinds of aberration: (1) point aberration, (2) distortion, and (3) chromatic aberration.

Point aberration produces a fuzzy image by failing to focus light to a point The three primary types of point aberration are (1) spherical aberration, (2) astigmatism, and (3) coma.

Spherical aberration occurs in a lens in which one or both sides are curved like a portion of the surface of a sphere. A spherical surface is easy and inexpensive to produce. As a result, a typical lens in a camera, a pair of binoculars, or a small telescope is a piece of glass with spherical surfaces. This kind of lens focuses light well enough for those applications.

Spherical aberration occurs in a lens because light rays that pass through the lens at different distances from its center come to a focus at different distances from its edge

To understand spherical aberration, imagine that parallel rays of light strike a lens as shown in the accompanying illustration. If there were no aberration, all the rays would come to a focus at a point on the other side of the lens. But because of aberration, rays that pass through different parts of it come to a focus at different points. All the points are located along the optical axis, an imaginary line through the center of the lens. Rays that pass through the lens near its center come to a focus at a point relatively far from the lens. Rays that pass through the lens near its edge come to a relatively close focus. An inward-curving spherical mirror produces spherical aberration in a similar way.

Aberrations are not necessarily a result of manufacturing errors. For example, even a lens with perfectly spherical sides could not focus all parallel rays that pass through it to a single point.

In astigmatism, light comes to a focus as a line, rather than a point. In coma, the light focuses in the shape of a cone. Astigmatism and coma make the image blurry at its edge but not its center.

Distortion results when a lens or mirror focuses light at an incorrect distance from the optical axis. The image will appear either stretched or compressed near the edges.

Chromatic aberration occurs in lenses but not in mirrors. It occurs because a lens focuses light rays of different colors at different distances. The distance to the focal point of a ray of light depends upon the wavelength of the light. Wavelength is the distance between successive crests of a light wave. Each color has its own wavelength. From the longest wavelength to the shortest, the colors of visible light include red, orange, yellow, green, blue, and violet.

Chromatic aberration occurs because a lens focuses different colors of light at different points – violet light at the nearest point red at the farthest, and the other colors at points in between

In longitudinal chromatic aberration, a lens focuses the different colors at different points on the optical axis. Violet light comes to a focus that is closest to the lens. Then comes the focal point for blue light, then the point for green, and so forth. In lateral chromatic aberration, focal points are also offset above and below the optical axis—red the least, violet the most.

Absorption and adsorption, are processes by which substances take in matter or energy, or both. In absorption, the matter or energy taken in becomes distributed throughout the absorbing material. Adsorption is the gathering of matter only. The matter collects on the surface of the adsorbing material. It does not enter the interior.

Absorption. There are many familiar examples of absorption. Heavy drapes absorb sound energy. The sound waves make the fibers in the drapes vibrate and rub together. Friction turns the sound energy into heat so the sound cannot be reflected as an echo (see Insulation [Insulation against sound]). Colored objects and filters selectively absorb light energy. White light is composed of all colors of light When white light strikes a colored substance, some colors of light energy are absorbed.

The absorbed light energy excites electrons in the atoms of the colored substance—that is, it raises them to higher energy levels. The substance will transmit or reflect the colors of light that have not been absorbed. Dry earth absorbs water by a process called capillarity {see Capillarity). The water in a lake absorbs oxygen by dissolving it.

Adsorption. Usually only solid material can act as an adsorbent The adsorbed matter can be the molecules of a liquid or a gas, or tiny particles of a solid. Adsorption is often highly selective, making it useful for separating or purifying liquids and gases. A charcoal filter adsorbs molecules on the surface of each charcoal particle. Silica gel adsorbs water molecules from moist air and holds them on the surface of each grain. Adsorption releases heat called heat of adsorption.

The interaction of adsorbed molecules with the adsorbing surface may be weak or strong. Molecules are weakly held when physical attraction holds them to the adsorbing surface (see Molecule [Molecules and matter]!. This process is called physiosorption. When adsorbed molecules are united chemically with the surface, they are strongly held. This process is called chemisorption. In chemisorption, the adsorbed molecules form a single layer. In physiosorption, they may form layers several molecules deep.