Diffraction, Interference, and Polarization

Abstract

Diffraction phenomena are essentially the same for electromagnetic waves as for sound waves. Thus the diagrams and relations developed for sound waves are applicable to radio waves. Short radio waves such as used in radar have much the same wavelengths as audible sound waves, so that even the scale of the patterns is similar. The phenomena are essentially the same for light waves too, but these electromagnetic waves are shorter than radar waves and sound waves by a factor of about a million, and as a result the manner of observing the pattern tends to be different. In dealing with sound waves, the aperture of a loudspeaker will not be more than a few wavelengths in diameter. The diameter of a radar antenna is somewhat greater than this, but rarely more than one hundred wavelengths in diameter. Apertures in optical systems, on the other hand, are very rarely less than one hundred times the wavelength. Scientists have grown accustomed, however, to accepting another limitation on the angular width of a beam of light, and that is the angular width of the incoherent source. In the 1960s, coherent sources of light became available. The power of such sources is increasing, and we can now approach the ultimate diffraction limitation of beamwidth for light as for microwave radio.