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.
22
Light Waves and Their Uses
inches’ focal length some twenty feet distant. About halfway a disc of about a quarter-inch diameter, and very smoothly and accurately turned, is suspended by three threads,1 so that its center is accurately in line with the pinhole and the center of the lens. The field of the lens will now be quite dark, except at the center of the shadow, where a bright point of light is seen.
We shall now attempt to show the analogue of the sound-shadow experiment by means of light waves. The light is
FIG. 18
concentrated on a very narrow slit A (Fig. 18), which may be supposed to act as the source of light waves. Another slit B, about an inch wide, is placed at a distance of about eight feet, and beyond this a screen C receives the light which has passed through B. The borders bb of the shadow of the slit B are quite sharply defined (though a very slight bending of the light around the edges may be observed by means of a lens focused on b). But if the slit be made narrow, as at B', the sharp boundary which should appear at cc is diffuse and colored, the light being bent into the geometrical shadow as indicated by the dotted lines. The narrower the second slit is made, the wider and more diffuse will be the image on the screen; that is to say, the greater will be the amount of bending into the shadow. An interesting variation of the experiment is made by using two slits instead of the second slit B. In this case, in addition to the
i The disc may be glued to a piece of optical glass, care beiDg taken that no trace of glue appears beyond the edge of the disc.