25.2

The Reflection of Light

Most objects reflect a certain portion of the light falling on them. Suppose a ray of light is incident on a flat, shiny surface, such as the mirror in Figure 25-3. As the drawing shows, the angle of incidence is the angle that the incident ray makes with respect to the normal, which is a line drawn perpendicular to the surface at the point of incidence. The angle of reflection is the angle that the reflected ray makes with the normal. The law of reflection describes the behavior of the incident and reflected rays.

Figure 25-3 The angle of reflection equals the angle of incidence . These angles are measured with respect to the normal, which is a line drawn perpendicular to the surface of the mirror at the point of incidence.

| Law of Reflection
| The incident ray, the reflected ray, and the normal to the surface all lie in the same plane, and the angle of reflection equals the angle of incidence :

When parallel light rays strike a smooth, plane surface, such as that in Figure 25-4a, the reflected rays are parallel to each other. This type of reflection is one example of what is known as specular reflection and is important in determining the properties of mirrors. Most surfaces, however, are not perfectly smooth, because they contain irregularities the sizes of which are equal to or greater than the wavelength of the light. The law of reflection applies to each ray, but the irregular surface reflects the light rays in various directions, as part b of the drawing suggests. This type of reflection is known as diffuse reflection. Common surfaces that give rise to diffuse reflection are most papers, wood, nonpolished metals, and walls covered with a “flat” (nongloss) paint.

Figure 25-4 (a) The drawing shows specular reflection from a polished plane surface, such as a mirror. The reflected rays are parallel to each other. (b) A rough surface reflects the light rays in all directions; this type of reflection is known as diffuse reflection.

A revolution in digital technology is occurring in the movie industry, where digital techniques are now being used to produce films. Until recently, films have been viewed primarily by using projectors that shine light directly through a strip of film containing the images. Now, however, projectors are available that allow a movie produced using digital techniques to be viewed completely without film by using a digital representation (zeros and ones) of the images. These digital projectors depend on the law of reflection and tiny mirrors called micromirrors, each about the size of one-fourth the diameter of a human hair. Each micromirror creates a tiny portion of an individual movie frame on the screen and serves as a pixel, like one of the glowing spots that comprise the picture on a TV screen or computer monitor. This pixel action is possible because a micromirror pivots about 10° in one direction or the reverse in response to the “zero” or “one” in the digital representation of the frame. One of the directions puts a portion of the light from a powerful xenon lamp on the screen, and the other does not. The pivoting action can occur as fast as 1000 times per second, leading to a series of light pulses for each pixel that the eye and the brain combine and interpret as a continuously changing image. Present-generation digital projectors use about 800 000 micromirrors to reproduce each of the three primary colors (red, green, and blue) that comprise a color image.

The physics of digital movie projectors and micromirrors.

Copyright © 2007 John Wiley & Sons, Inc. All rights reserved.