Wave Properties Of Light
Light traveling through empty space moves in a straight line. In our everyday experience we encounter light not in empty space but passing through various media-light partially absorbed by the atmosphere, scattered by dust, transmitted through a window or a telescope. In these circumstances the speed of light may be slowed and the direction of the light wave may be changed. These changes are best understood through the wave properties of light.
Several properties illustrate the wave characteristics of light. One is reflection, which occurs when light strikes the boundary between two media of different materials, such as glass and air. When a light ray moving in air reaches the boundary, part of it may be reflected. The reflected ray lies in the plane formed by the incident ray and the perpendicular to the boundary. The ordinary mirror, or looking glass, illustrates reflection.
Also, part of the incident ray may be transmitted through the glass rather than being reflected. The transmitted ray does not, however, continue along the same straight line; it is bent toward the perpendicular. This change in direction is called refraction. If the medium into which the ray moves is more dense than that from which it comes, the angle of refraction will be less than the angle of incidence. If its density is less, then the angle of refraction is greater. A good example of refraction is a spoon sticking out of a glass of water. The handle looks bent at the point where the spoon enters the water because part of the handle is in the same medium (air) as you, while for the part under water light must pass through the water-air boundary, where it is refracted.
Light shows another wave property, diffraction, which is the spreading out of light past the edges of an opaque body. Instead of being propagated in a straight line, light, like sound waves, bends around corners. The spread is greater for longer wavelengths. Because light's wavelength is very small, we do not normally observe diffraction in the everyday world. We can see diffraction, though, in the laboratory. Optical instruments, such as telescopes and microscopes, depend upon these wave properties of light for their operation.
Nearly all natural light sources, such as stars, emit electromagnetic waves composed of many wavelengths. How do waves of different wavelengths add to produce a composite wave? If waves of the same wavelength from two sources are superimposed so that their crests and troughs coincide, they are said to be in phase with each other, and their amplitudes add to produce a sum greater than the amplitudes of the individual waves; the light is said to "interfere constructively." If the crests of one set of waves fallon the troughs of the other, they are said to be out of phase with each other, and their amplitudes cancel each other; the light is said to "interfere destructively." interference is common to all types of waves; in fact occurrence was strong evidence that light is a wave ohenomenon. Light waves of one or many different wavelengths may interfere constructively or destructively. Such waves are called composite waves, or white light, since that is the physiological response they evoke. If we can add waves together, then we must also be able to separate a composite wave into its constit'uent wavelengths.
