20
Light Waves and Their Uses
This flame is produced by allowing a jet of gas to issue under considerable pressure from a small nozzle, and by gradually increasing the pressure until the flame is on the point of flaring. On blowing the whistle, we observe that the flame ducks; it is lowered to perhaps one-third or one-fourth of its height, and broadens out at the same time. On placing the whistle behind an obstacle, we observe by the ducking of the flame that it responds to the whistle almost as readily as when no obstacle was present.
I now take a shorter whistle, half an inch long; which, therefore, produces a sound wave two inches long. The flame responds even more readily to this than to the longer whistle, and when the shorter whistle is sounded behind the obstacle the flame ducks, but to a much less marked degree than l>efore.
I have here the means of producing still higher sounds. Strung on a piece of wire are a number of iron washers — rings of iron about an inch in diameter. When these are shaken they produce vibrations whose wave length is even shorter than that produced by the whistle just sounded. On shaking the rings you perceive the immediate response of the flame, and on shaking the rings behind the obstacle the flame responds still, but much more feebly. I take a new set of rings one-half inch in diameter. On shaking these the flame responds as before, but when I place the rings behind the obstacle the flame scarcely responds at all. I take a still smaller series of discs. These are approximately only one-fourth of an inch in diameter and produce a wave whose length is approximately one-lialf inch. On shaking the last set of discs outside the obstacle the flame responds not quite so strongly as before, because the total amount of energy in this case is very small; but, on shaking the discs behind the obstacle, the flame is absolutely quiescent, showing that the sound shadow is perfect. In moving the discs
Microscope, Telescope, Interferometer 21
to and fro while shaking them, the geometrical limit of the shadow can be definitely marked to within something like half an inch; that is, a quantity of the same order as the length of the sound wave which is being used.
It is evident from the foregoing that, if we wish to investigate the bending of light waves around a shadow, we must take into account the fact which has already been established, namely, that the light waves themselves are exceedingly small — something of the order of a fifty-thousandth of an inch. The corresponding bending around an obstacle might, therefore, be expected to be a quantity of this same order; hence, in order to observe this effect, special means would have to be adopted for magnifying it.
The diffraction of sound waves is beautifully shown by the following experiment:1 A bird call is sounded about ten feet from a sensitive flame, and a circular disc of glass about a foot in diameter is interposed. If the adjustment is imperfect, the sound waves are completely cut off; but when the centering of the plate is exact, the sound waves are just as efficient as though the obstacle were removed.
This surprising result was first indicated by Poisson, and was considered a very serious objection to the undulatory theory of light. It was naturally considered absurd to say that in the very center of a geometrical shadow there should not only be light, but that the brightness should be fully as great as though no obstacle were present. The experiment was actually tried, however, and abundantly confirmed the remarkable prediction.
The experiment cannot be shown to an audience by projecting on a screen, but an individual need have no difficulty in observing the effect. The image of an arc light (or, better, of the sun) is concentrated on a pinhole in a card, and the light passing through is observed by a lens of two or three
l Exhibited by Lord Rayleigh at the Royal Institute.