(1) Light, Its Rectilinear Propagation, Shadows
352. A Comparison of Sound and Light.—Light from the standpoint of physics is considered much as is sound, as a mode of motion; one affecting the ear, the other producing the result called vision. There are other differences also worth considering. (a) While sound travels as vibrations of some material medium, light travels only as vibrations of the ether; solids, liquids, and gases act so as to hinder rather than to assist in its movement. That is, light travels best in a vacuum or in a space devoid of ordinary matter. (b) The speed of light is so great that at ordinary distances on the earth its motion is practically instantaneous. Experiments have shown that its speed is about 186,000 miles to 300,000 kilometers a second.
353. Luminous and Illuminated Bodies.—If we consider the objects within a room, some of them, as books and furniture, would be invisible if all light from external sources were excluded. On the other hand, some other objects, such as a lighted lamp, a burning coal, or a red hot iron, would be seen if no outside light were present. Such bodies are said to be luminous. Most luminous bodies are hot and become non-luminous on cooling. There are, however, some bodies that are luminous at ordinary room temperatures, as the firefly and some phosphorescent paints. When light emitted by a luminous body strikes an object, a portion of it is always reflected. It is this reflected light that makes the illuminated object visible. If the object is a sheet of glass, some of the light is transmitted. If a substance is so clear that objects can be seen through it, the substance is transparent, but if objects cannot be seen through it, the substance is said to be translucent. Objects transmitting no light are opaque. Some of the light falling upon a body is neither reflected nor transmitted, but is absorbed and tends to warm the body. The light falling upon a body is therefore either reflected, transmitted, or absorbed. Thus Fig. 345 represents light coming from S to a piece of glass GL. A portion of the light represented by R is reflected. Another part A is absorbed and disappears, while still another part T is transmitted and passes on.
Fig. 345.—The light is transmitted (T), reflected (R), or absorbed (A).
There is no sharply drawn line between transparent and opaque bodies. Very thin sheets of gold transmit a greenish light, and experiments have shown that substances as transparent as clear water absorb enough light so that at considerable depths in an ocean or lake little or no light is ever found. All light whether from luminous bodies or reflected from non-luminous objects shows certain properties which will now be considered.
354. The Rectilinear Propagation of Light.—If a beam of light passes through a hole in a window shade into a darkened room, it is seen to follow a perfectly straight course. If a person while coughing holds a book before the face, the sound passes around the book and is heard at any point in the room while the face is hidden by the book. In other words, light ordinarily does not pass around corners as sound does, but travels in straight lines. This fact is made use of when one aims a gun or merely looks at an object. So well established in our minds is the idea that an object is in the direction from which we see the light coming to us from it, that we are sometimes deceived as to the real position of an object, when the course of the light from it has been changed by a mirror or some other reflecting surface. Many illusions are produced in this way, of which the mirage of the desert is one example. (See Art. 381.)
Fig. 346.—Shadow from a small source of light.
