Difraction

The bending of waves when they pass near the edge of an obstacle or through small openings is called diffraction. The fact that it can be observed for light under proper conditions is strong evidence in favour of the wave theory. The iridescent rainbow play of color that you see when white light is reflected almost parallel to the surface from a gramophone record is due to the fact that the various wavelengths of light are diffracted by different amounts when reflected by the regularly spaced ridges with which the surface is covered. In fact, a surface covered by fine, evenly spaced channels or ridges can be used as a substitute for the prism in a spectroscope. Such diffraction gratings are made by special machines that rule extremely fine scratched on metal or glass plates by means of a diamond point. A good grating of this kind may have 6000 or more rulings to the centimeter and is capable of giving much greater dispersion than any prism. As fine as optical gratings are, they are too coarse for producing diffraction of the very much shorter wavelength X-rays. But crystals of certain minerals can serve as natural gratings for this purpose. The regular spacing of the atoms in a crystal is just of the right order of size for diffracting X-rays and thus can serve to measure their wavelengths. Then using X-rays of known wavelength, the exact arrangement of the atoms in other crystals can often be worked out.