UNIT VI SECTION 5 LIGHT MAY BE REFLECTED AND REFRACTED Introduction. The reflection and refraction of light have many important applications today. Many of our most important scientific instruments, such as the telescope and the microscope, depend upon these properties of light. A Few Terms Used in Connection with the Propagation of Light Are Defined. Light travels through space without loss of energy, but when it travels through matter such as water or glass, a small amount of energy is absorbed ; such materials are said to be transparent. Some materials, like frosted glass, which scatter most of the incident light, are said to be translucent. Other materials, like wood or stone, which transmit no light, are said to be opaque. When light is thrown back upon striking a surface, it is said to be reflected. The ray striking the surface is called the incident ray, and the ray leaving the surface is called the reflected ray. That portion of light which is lost when a beam of light passes through a transparent medium or when it is reflected from a medium is said to be absorbed. Light which is absorbed is changed into some other form of energy such as chemical energy, electrical energy, or heat energy. When light changes its direction as it passes from one medium to another, it is said to be refracted. When light strikes the edge of a body or passes through a small aperture, the light does not cast a sharp shadow because a portion of the light deviates from its course, i.e., it spreads out. This spreading-out of light is called diffraction. The diffusion, or scattering, of light is different from diffraction in that it is the result of multiple reflections from rough surfaces. Light Travels in a Straight Line. Light travels in a straight line in a vacuum or in any homogeneous transparent medium. The path of a projector light in a smoke-filled 432
LIGHT MAY BE REFLECTED AND REFRACTED 433 theatre demonstrates the path of Hght by the illumination of the smoke particles which the air contains. The Reflection of Light Has Some Interesting Modem Applications. Some objects, such as incandescent lights, are luminous and are themselves the sources of the light which reaches the eye; but the majority of objects are visible only because of the light which they reflect to the eye. A perfect mirror would reflect all of the light falling upon it and would in itself not be visible, while a rough surface reflects some of the light and absorbs some of the light which falls upon it. The reflected light is reflected from the rough surface in many directions and is thus diffused. Under such conditions the surface becomes visible. When a ray of light strikes a mirror, it will be observed that the incident and reflected rays make the same angle with a line perpendicular to the mirror at the point of incidence. This is the fundamental law of reflection. If two mirrors are placed at right angles to each other, the rays will be reflected in a line parallel to that in which they came, Fig. 200. The angle of incidence equals the angle of reflection. (From Optics and Wheels, Courtesy of the General Motors Corporation.) regardless of the angle at which the incident rays reach the first mirror. If three mirrors are placed perpendicular to each other, as shown in Fig. 201, all light rays will be reflected back in the direction from which they came, regardless of how the combination of mirrors is held. Fig. 201. Reflection from three mirrors at right angles. The entering and returning rays are parallel, regardless of the position of the mirrors relative to the direction of the rays.