411. The Wave Theory of Light.—The theory that light is a form of wave motion was first advanced by Huygens, a Dutch physicist, in the seventeenth century. This theory was opposed at the start since (A) no medium was known to exist which would convey wave motion through space, as from the sun to the earth, and (B) the rectilinear motion of light was unlike that of any other form of known wave motions, such as that of water or of sound waves which are able to bend around corners. In answer to the first objection, Huygens assumed the presence of a medium which he named ether, while the second objection has been completely overcome during the past century by the discovery that light may deviate from a straight line. It is now known that the excessive shortness of light waves is the reason for its straight-line motion. Further, long ether waves, as those of wireless telegraphy, are found to bend around obstacles in a manner similar to those of water or sound.
Fig. 409.—Two plates pressed together by a screw clamp.
Fig. 410.—Illustrating the interference of light by a thin film of air.
412. The interference of light is one of the phenomena for which the wave theory offers the only satisfactory explanation. Interference of light may be shown by taking two pieces of plate glass and forcibly pressing them together by a screw clamp, as shown in Fig. 409. After a certain pressure has been reached, colored rings will appear about the compressed spot when viewed by light reflected from the upper surface of the glass. If light of one color, such as that transmitted by red glass, falls upon the apparatus, the rings are seen to be alternately red and dark bands. The explanation of this phenomenon according to the wave theory is as follows: The two sheets of glass, although tightly pressed together, are separated in most places by a thin wedge of air (see Fig. 410), which represents in an exaggerated form the bending of the plates when pressed by the clamp. Several waves are represented as coming from the right and entering the glass. Now the wave moving from R to the plates has some of its light reflected from each glass surface. Consider the two portions of the wave reflected at each of the surfaces between the plates, i.e., from the two surfaces of the wedge of air. If the portion of the wave reflected from the second surface of the air wedge combines with that reflected from the first surface, in the same phase as at C, the two reflected waves strengthen each other. While if the two reflected portions of the wave meet in opposite phases as at A and B, a decrease or a complete extinction of the light results. This is called interference. If light of one wave length is used, as red light, the regions of reinforcement and interference are shown by red and dark rings, while if white light is used, the ring where red light interferes, yields its complementary color, greenish blue. Where interference of greenish blue occurs, red is found, etc. Many phenomena are due to interference, such as (A) the color of thin films of oil on water, where the portions of light reflected from the two surfaces of the oil film interfere resulting in the production of color; (B) the color of soap bubbles. When first formed, soap-bubble films are not thin enough to show interference well, but as the bubbles increase in size or become thinner on standing, the conditions for interference are reached and, as the film becomes thinner, a regular succession of colors is noticed.
413. Differences Between Light and Sound.—Among the important differences between light and sound that have been considered are the following: the former are (a) waves in the ether, (b) of very short wave length, and (c) their motion is in straight lines. Another difference (d) is in the mode of vibration.
Sound waves are longitudinal, while light waves are transverse. Light waves consist of vibrations of the ether at right angles to the line of motion. To illustrate the reasoning that has led to this conclusion, suppose a rope to be passed through two vertical gratings. (See Fig. 411, 1.) If the rope be set in transverse vibration by a hand, the waves produced will readily pass through to the gratings P and Q and continue in the part extending beyond Q. If, however, Q is at right angles to P, no motion will be found beyond Q. Now if a stretched coiled spring with longitudinal vibrations should take the place of the rope, it is evident that the crossed position of the two gratings would offer no obstacles to the movement of the vibration. In other words, crossed gratings offer no obstruction to longitudinal vibrations, while they may completely stop transverse vibrations.
Fig. 411.—Transverse waves will pass through both gratings in (1) where the openings in the two gratings are at right angles. The waves passing P are stopped by Q (2).Fig. 411.—Transverse waves will pass through both gratings in (1) where the openings in the two gratings are at right angles. The waves passing P are stopped by Q (2).
Fig. 412.—Effect of tourmaline crystals on light.