Notebook, 1993-

LIGHT & COLOR - Optical Instruments

The following is from: Light and Color, by Clarence Rainwater, Prof. of Physics, San Francisco State College, Original Project Editor Herbert S. Zim, Golden Press, NY, Western Publishing Company, Inc., 1971.

Light & Color as Tools

Lasers produce an intense, monochromatic beam of light that can be focused to weld, melt, or vaporize a small amount of any substance. They may be used in communications, in the acceleration of chemical reactions, and in surgery. Laser is an acronym for "light amplification by stimulated emission of radiation." The first laser was built in 1960.

Lasers emit coherent (in-step) light waves, whereas an ordinary light source radiates light of mixed wavelengths in an out-of-step (noncoherent) and random manner. The luminance of the image of ordinary light cannot exceed the source's luminance. But a laser can form a very luminant image because its parallel rays can be focused to a tiny spot and are added together in phase.

The light from a small laser, in fact, can be focused to form an image of luminance greater than that of the sun's surface. The concentration of energy is so great that extremely high temperatures are produced. Light rays from a laser can be beamed through space with a fraction of an inch spread per mile. The light is also extremely pure in color (monochromatic).

In a Ruby Laser a ruby crystal rod has plane parallel polished ends which are silvered like mirrors. One end is only partially silvered and acts as a window for the light to get out. Energy is supplied to the ruby crystal by a powerful flash tube lamp. This serves to pump the (chromium) atoms of the crystal to a "metastable" energy state in which they linger for a few thousandths of a second before dropping to the ground state with the emission of a photon of light. Most of these photons pass out of the crystal walls and are lost, but soon one photon will move directly along the rod and is reflected from the polished ends, passing back and forth along the rod until it encounters an atom in the excited metastable state. The excited atom than radiates its photon in exact phase with the photon which struck it. This second photon may in turn stimulate another atom, and this "cascade" process continues until the whole crystal is filled with in-phase radiation oscillating back and forth inside the rod. Part of this radiation is emitted through the half-silvered end of the rod and becomes the laser beam. All of this takes place within a few billionths of a second, then the flash tube fires again and the process is repeated.

Modern lasers have been made of solid crystals, glass, liquids and gases. Some operate in pulses as described, but many emit continuously. In all, the radiation is monochromatic and coherent. It is this high degrees of coherence that makes laser light different from that of all other sources. [p. 152-53]

Holography is a special kind of photography in which the film captures not the image of the subject but the pattern of the wavefronts of light reflected from the subject. Invented by Dennis Gabor in 1948, the process took on new importance when the laser was invented. In addition to its uses in the entertainment field, holography has many scientific and industrial applications, which are being developed rapidly.

To make a hologram, a coherent light beam (from a laser) is split into two parts--an object beam which illuminates the subject, and a reference beam which is directed to the photographic film by mirrors. At the film the light reflected from the subject interferes with the reference beam to cause a complex pattern of light and dark fringes in the film when it is developed. When a hologram is held in a beam of coherent light, part of the diffracted light is a reproduction of the original light wave pattern that came from the subject. Thus, a viewer looking through the hologram can see a life-like reproduction of the original scene. This virtual image is truly three-dimensional, since the viewer can move his head and see a different perspective of the subject. Besides the virtual image, there is also a real image formed by the rays diffracted in another direction. [p. 154]

Fiber Optics is the result of an ingenious application of a simple principle. Imagine a light ray that has entered the end of a slim solid glass rod. If it always strikes the surface of the glass at angles greater than the critical angle, the ray will be totally reflected each time and trapped within the glass. Reflected from side to side, the light ray will be conducted along the rod like water in a hose. Finally, the light ray hits the end of the rod at a small angle to the perpendicular and is able to exit. This explains the success of bent lucite and glass rods as light-conductors in advertising displays and in simple illuminating devices.

A bundle of fibers produces a grainy image like a halftone printing process. By drawing out the fibers so that one end of the bundle is smaller than the other, the image can be enlarged or reduced and its brightness changed. The quality of the image can be improved by shaping the end of the bundle or by adjusting the alignment of the fibers to eliminate distortions. [p. 155-56]

A big advance in fiber optics came with the development of very fine clear fibers encased in a thin coating of lower refractive index. Total reflection takes place between the fiber and its coating. The fibers are so thin (a few microns in diameter) that they are flexible, and their coatings allow them to be bound into bundles without interfering with each other's action. Such a bundle can conduct light for several feet. When the fibers are lined up so they have the same relative position at each end of the bundle, they can transmit images. This makes possible a sort of super-periscope so flexible that physicians can use it to examine the interior of body organs. A sheath of unaligned fibers around the bundle can carry illumination to the area being examined.

R  E  F  E  R  E  N  C  E  S 
[Light and Color, by Clarence Rainwater, Prof. of Physics, San Francisco State College, Original Project Editor Herbert S. Zim, Golden Press, NY, Western Publishing Company, Inc., 1971.]



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