Stranica 72 [72]
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).