If a large opaque screen ML is placed near a point-source (0) as in Fig. 14, rays drawn grazing its boundary form what is called the shadow.' Thus if OMP is one of there rays, the shadow lies on one side of it. The effect at a point Q just within the shadow can he found by considering the zones on the wave-front around which correspond to Q. it is evident that there is an action at Q due to those zones which are not obscured by the ob stacle. As points are taken farther and farther in the shadow, however, this effect becomes less and less, gradually fading away. (This explains why atrial waves pass around Corners into the 'shadow,' and asserts the same phenomena for ether-waves, only to a much less distance, owing to the smallness of the zones in their case. This is actually observed.) Similarly, the action at a point 1: outside the shadow depend, it the number of zones drawn around S, the intersection of (III with the wave front, which are not obscured by the opaque ob stacle. If 11 is very close to B. only the first zone may he uneovered. and the action is intense: but as R recedes from P the second zone begins to neutralize the action of the first, and so the in tensity at 1: decreases; then, as lt continues to recede, the action again increases. etc.. until R is so far away from P that the action of the zones of high order makes no difference. Conse quently, if homogeneous light is used, and a screen is placed to receive the shadow, there will be no sharp shadow, hut the light will fade away in the geometrical shadow, while out side this shadow there will he bright and dark bands following the general shape of the obstacle. These phenomena can he easily observed. If white light is used. these rings will be colored, because their position depends upon the wave-length and so each color will have its own set of rings; but at a comparatively short distanee from the edge of the geometrical shadow the effect is a uniform white illumination. These hands are called 'dif fraction' hands; and the whole phenomenon is said to he due to 'diffraction' (q.v.). as is also that of the small disks and openings.
These diffraction rings or bands are seen most clearly if the opaque obstacle has a linear edge like a knife edge and if it is illuminated by light coming through a slit parallel to this edge: then, if the light is received on a screen suitably placed, the bands will be most distinct. Other
cases of diffraction will be discussed later.
it thus appears that the sense in which light travels in straight lines is not in the casting of sharp shadows, for it does not, but in the fact that as ether-waves spread out from a point source 0. there being no obstacles, the effect at a point P is due entirely to the action of a minute portion of the wave-front around Al where the ray OP intersects it.
LA•s; OF REFLECTION. It is not difficult to show that the laws of reflection are a direct consequence of the principle of the rectilinear propagation of light. Let AB'C'A' he the section of a plane surface by the paper; draw the line ABC to represent the section of a plane by the paper, and similarly the line A'C'"13", making the angles CAA' and C"A'A equal; draw the lines CC' and BB' perpendicular to the plane ABC and the lines CC" and B'B" perpendicular to the plane A'C"B": draw also 13T perpendicu lar to CC' and C'Q perpendicular to 13'B". If the plane A'C"B" represents a wave-front reced ing from the plane surface the actions at 13" and C" are entirely due to those that were at B' and C' a short time previously; similarly, if the plane ABC represents an advancing wave front, the actions at B' and C" will be due to now at the points 13 and C. Therefore if the time taken for the disturbance to pass from B to it to It" is the same as it is for the dis turbance to pass from C to C' to C", the receding wave-front is due to the reflection at the surface A13'C'A' of a wave-front which was at ABC some time previously. But these times are the same because the lengths of the lines CC"C" and B13'13" are evidently equal by geometry. and the me dium in which the disturbances are traveling is the same for both. Consequently, the ray B13' is reflected into the ray 13'B"; they make equal angles With a line drawn perpendicular to the surface at 13' and these lines lie in one plane.