Stranica 72 [72]
OCR
40 LIGHT AND WATER travelling to or from the spectator, may be illustrated by a rough experiment, such as the following. A sheet of glass is bedaubed on one side with vaseline or other grease, and then carefully drawn, from right to left or vice versá, at right angles to their former direction. Suppose the curve in Fig. 16 to represent the section of the waves through X (Fig. 14) now Q running in the transverse direction, but of the same height and length as before. Q and P occupy the same positions, and are here shown in elevation, the vertical line PX representing the plane of the paper in Fig. 14. The normal to the surface of the water at any point would reach this plane at N or thereabouts (the position of N Fig. 16. being deduced from Fig. 14). By drawing from N a line NK at the greatest inclination that the normal can assume (in this case 23° with the vertical), we get the point K as the farthest possible point to the right of PX at which P can be seen reflected. Figs. 14, 15 and 16 all being drawn to the same scale, we now see how small is the lateral shifting of the image in comparison with the vertical displacement. In choppy water the normals to the surface point in all directions within a certain limit, and the line of light becomes, as we have said, widened into a streak, but the greater the distance from the central line, the less the number of points that catch the light, so that the streak has no definite edges and fades very gradually away. | If, in Fig. 16, we suppose the luminous point to be lowered to P, we get KN for the normal, and the position of K now shows the limit of possible reflexion of P’. Thus, as the position of the object (or the observer) is lowered, the streak narrows. This we have already seen to be the case with the path of light below the sun (Plates IX and X).
