Page:The American Cyclopædia (1879) Volume X.djvu/444

 438 LIGHT FIG. 3. light. In regard to the effect resulting from obliquity of the receiving surface, the law is that the intensity of light received is propor- tional to the cosine of the angle which the in- cident rays make with a perpendicular to the surface. Let a &, fig. 3, be a surface receiving a beam of parallel rays, clad. The quantity of light fall- ing on a & will be the same as that which would fall upon a sur- face e 5, perpendicu- lar to c & and d e; therefore the intensi- ty of light received by these surfaces is in- versely proportional to their areas. But e &=a & x cos. a 6 e=a & x cos. c 6 y, the angle included between the in- cident rays and the perpendicular to the surface. The same law has been applied to heat, and is true as to the number of rays falling upon the surface, but not strictly so as to the amount of heat absorbed. (See HEAT.) The intensity of light emitted from a luminous surface obeys the same law. For the comparison of the in- tensity of different sources of light, see PHO- TOMETEY. Absorption and Emission. In re- gard to the properties of bodies by which they allow light to be transmitted through them, or cause its absorption, they are classified as transparent, translucent, and opaque. Trans- parent bodies are those which transmit light with little or no perceptible loss, and through which objects may be distinguished. Trans- lucent bodies allow much of the light to pass through them, but prevent objects from being viewed through them, such as ground glass, oiled paper, and horn. Opaque bodies are those which absorb the rays of light, or pre- vent most of them from passing through them. These properties of bodies depend upon their molecular constitution. Some bodies have the power of transmitting the non-luminous but not the luminous rays of the spectrum; such are called diathermanous bodies. (See DIA- THERMANCY.) Dry air and rock salt are bodies that are almost perfectly transparent as well as diathermanous; or, as it is sometimes said, transparent to all the rays, visible and invisible. Kock salt is the only known solid having this property nearly perfect. Those bodies which permit the ether within them to transmit un- dulations of medium wave length from and to the ether around them are transparent to the luminous rays; those whose molecular constitution causes the ether undulations to be broken up are opaque. There are no bodies which are perfectly opaque, as is shown by cutting them in very thin slices, or hammering them into thin films, when most of them will be found slightly translucent. Foucault has coat- ed the object glass of a telescope with so thin a film of silver that the sun can be viewed through it. Reflection and Refraction. When a beam of light meets the surface of a new me- dium, a portion of it is always turned back or reflected, while another portion is propaga- ted onward in the second medium, and is also turned out of its original course, or refracted. The angles which are made by the incident and reflected rays with a perpendicular to the sur- face are called the angles of incidence and re- flection respectively, and are always equal to each other. Light is said to be regularly and irregularly reflected. The image formed in a mirror is regularly reflected, but the rougher surfaces of ordinary objects reflect light irregu- larly in all directions without forming an im- age. The intensity of reflected light varies with the reflecting surface and with its posi- tion. The differences also in the reflecting powers of different substances are greater for small than for large angles of incidence. Thus water reflects only -^ part of a perpendicular beam, while mercury reflects two thirds; but when the incident angle is 89^-, they each re- flect -&VTT of the incident light. The refract- ed rays are deflected in a direction either to or from the perpendicular, depending upon the re- fracting power of the second medium. When its refracting power is greater, the direction is toward the perpendicular, and when it is less T from it. Ptolemy measured the refraction of glass and water at various angles, and he ob- served that the angle of incidence was greater than the angle of refraction; but he erred in supposing the proportion to be invariable for different angles, it being left for Willebrord Snell, about 1621, to demonstrate that in re- fraction there is an exact proportion between the sines of the angles of incidence and re- fraction, instead of between the angles them- selves. Alhazen had long before shown that the angles vary as the incident rays are more or less oblique, but failed to discover the natural law. Alhazen's discovery, however, had not prevented mathematicians from gen- erally adhering to the old notion of Ptolemy down to the time of Kepler, who again saw the error, and published an approximate cor- rection in 1604. The laws of single refraction may be stated as follows: 1. At any angle of incidence the ratio of the sines of the angles of incidence and refraction is constant for the same two media, but varies with differ- ent media, and this ratio is called the index of refraction. 2. The incident and the re- fracted rays are in the same plane, which is perpendicular to the plane separating the two media. These have been generally known as Descartes's laws, but, as stated above, their discovery is due to Willebrord Snell. The in- dex of refraction, as will be seen further on, also varies somewhat with the nature of the light, whether red, green, or violet; and in exact experiments homogeneous light alone is used, but this does not affect the general law. The index of refraction from air to water is , and is called the index of refraction for water. In calculating the indices of refraction for me-