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The change of direction that occurs when a ray of light passes at an oblique angle (less than 90°) from one transparent substance INTO another substance of different density is called refraction.

Refraction occurs because light travels at various speeds in different transparent substances of different densities. The greater the density of a substance, the slower the light travels through it.

Refraction (or change of direction) always follows a simple rule: when the light ray passes from one transparent substance into another of greater density, refraction is toward the normal. In this context, the normal means a line

Figure 5-3-5.—Reflected light. (A) Regular (specular); (B) Irregular (diffused).

Figure 5-3-6.— Wave front diagram illustrating refraction of light at an air-water boundary. Ray is entering a more dense substanc 

Perpendicular to the surface of the medium at the point of entrance of the light ray. (See fig. 5-3-6.) In passing from one transparent substance into another of lesser density, refraction is away from the normal. (See fig. 5-3-7.)

Figure 5-3-7.—Wave front diagram illustrating refraction of light at an air-water boundary. Ray is entering a less dense substance. 

When a ray of light enters a denser medium at an angle of 90°, as shown in figure 5-3-8, the wave fronts slow down but remain parallel. When this same light ray enters a in the air. Consequently, the ray bends toward the normal. (See fig. 5-3-6.) If the light ray enters a less dense medium, at an oblique angle, the ray bends away from the normal as shown in figure 5-3-7. The portion of the wave front that enters the less dense substance travels faster than the other part of the wave front. Consequently, the ray bends away from the normal.

When a beam of white light is passed through a prism, as shown in figure 5-3-2, it is refracted and dispersed into its component wavelengths. Each of these wavelengths reacts differently on the eye, which then sees the various colors that compose the visible spectrum. 

The visible spectrum ranges in color from violet at one end to red at the other end. (See fig. 5-3-2.) There are six distinct colors in the spec-trum: red, orange, yellow, green, blue, and violet. However, a mixture of these colors is also present.

Learning Objective: Identify the characteristics of photometers (halos, coronas, rainbows, fogbows, mirages, looming, scintillation and crepuscular rays).

Figure 5-3-8.—Wave front diagram illustrating the difference in the speed of light in air and water.

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