=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. [Illustration: FIG. 409.--Two plates pressed together by a screw clamp.] [Illustration: 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.