HENRY A. ROWLAND, JOHNS HOPKINS Distinguished for the invention of the concave grating and for epoch-making studies in heat and electricity SIR WILLIAM CROOKES, LONDON Distinguished for his pioneer work (1875) in the study and interpretation of cathode rays (pp. 438 and 443) A GROUP OF MODERN PHYSICISTS X-RAY PICTURE OF THE HUMAN THORAX This figure is a remarkable picture of the human thorax with the apex of the heart showing clearly on the right of the spinal column and the base stretching across the column, part of it showing distinctly on the left side opposite the apex 421. Diffusion of light. In the last experiment the light was reflected by a very smooth plane surface. Now let the beam be allowed to fall upon a rough surface like that of a sheet of unglazed white paper. No reflected beam will be seen; but instead the whole room will be brightened appreciably, so that the outline of objects before invisible may be plainly distinguished. The beam has evidently been scattered in all directions by the innumerable little reflecting surfaces of which the surface of the paper is composed. The effect will be much more noticeable if the beam is allowed to fall alternately on a piece of deadblack cloth and on the white paper. FIG. 378. Regular and irregular reflection The light is largely absorbed by the cloth, while it is scattered or diffusely reflected by the paper. Illumination sufficiently strong for sewing on white material may be altogether too weak for working on black goods. The difference between a smooth reflector and a rough one is illustrated in greatly magnified form in Fig. 378. The air shafts of apartment houses are made white to get the maximum diffusion of daylight into rooms that might otherwise be very dark. 422. Visibility of nonluminous bodies. Everyone is familiar with the fact that certain classes of bodies, such as the sun, a gas flame, etc., are self-luminous (that is, visible on their own account), while other bodies, like books, chairs, tables, etc., can be seen only when they are in the presence of luminous bodies. The above experiment shows how such nonluminous, diffusing bodies become visible in the presence of luminous bodies. For, since a diffusing surface scatters in all directions the light which falls upon it, each small element of such a surface is sending out light in a great many directions, in much the same way in which each point on a luminous surface is sending 2 out light in all directions. Hence we always see the outline of a diffusing surface as we do that of an emitting surface, no matter where the eye is placed. On the other hand, when light comes to the eye from a polished reflecting surface, since the form of the beam is wholly undisturbed by the reflection, we see the outline not of the mirror but rather of the source from which the light came to the mirror, whether this source is itself self-luminous or not. All bodies other than selfluminous ones are visible only by the light which they diffuse. Black bodies send no light to the eye, but their outlines can be distinguished by the light which comes from the background Any object which can be seen, therefore, may be regarded as itself sending rays to the eye; that is, it may be treated as a luminous body. 423. Refraction. Let a narrow beam of sunlight be allowed to fall on a thick glass plate with a polished front and whitened back* (Fig. 379). It will be seen to split into a reflected and a transmitted portion. The transmitted portion will be seen to be bent toward the perpendicular OP drawn into the glass. Upon emergence into the air it will be bent again, but this time away from the perpendicular O'P' drawn into the air. Let the incident beam strike the surface at different angles. It will be seen that the greater the angle of incidence the greater the bending. At normal incidence there will be no bending at all. If the upper and lower faces of the glass are parallel, the bending at the two faces will always be the same, so that the emergent beam is parallel to the incident beam. 'P' FIG. 379. Path of a ray through a *All of these experiments on reflection and refraction may be done effectively and conveniently by using disks of glass, like those used with the Hartl Optical Disk, through which the beam can be traced. Similar experiments made with other substances have brought out the general law that whenever light travels obliquely from one medium into another in which the speed is less it is bent toward the perpendicular) and when it passes from one medium to another in which the speed is greater it is bent away from the perpendicular, drawn into the second medium. 424. Total reflection; critical angle. Since rays emerging from a medium like water into one of less density, like air, are always A B source I under water to the boundary between air and water at different angles of incidence bent from the perpendicular (see FIG. 380. Rays coming from a IlA, ImB, etc., Fig. 380), it is clear that if the angle of incidence on the under surface of the water is made larger and larger, a point must be reached at which the refracted ray is parallel to the surface (see In C, Fig. 380). It is interesting to inquire what will happen to a ray Io which strikes the surface at a still greater angle of incidence IoP'. It will not be unnatural to suppose that since the ray nC just grazed the surface, the ray Io will not be able to emerge at all. The following experiment will show that this is indeed the case. B -P FIG. 381. Transmission and reflection of light at surface AB of a right-angled prism Let a prism with three polished edges, a polished front, and a whitened back be held in the path of a narrow beam of sunlight, as shown in Fig. 381. If the angle of incidence IOP is small, the beam will divide at O into a reflected and a transmitted portion, the former going to S', the latter to S (neglect the color for the rotated slowly in the direction of the arrow. A point will be reached at which the transmitted beam disappears completely, while at the same time the spot at S' shows an appreciable increase in brightness. Since present). Let the prism be |