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The phenomena of interference and diffraction show that light is propagated as a wave motion, but they do not show whether light is a longitudinal wave or a transverse wave. The fact that the velocity of light is the same as the velocity of radio waves and the radiation of visible light from accelerated electrons, as in a betatron, indicates that light is an electromagnetic wave. We recall from Section 20-9 that a wave can be shown to be transverse if a device can be found which will prevent passage of the wave in one orientation and will allow the wave to be transmitted when in a second orientation at right angles to the first, as in the case of a slit and a transverse wave on a string. Since a longitudinal wave will pass through a slit, however that slit is oriented, longitudinal waves may be distinguished from transverse waves by our inability to demonstrate the property of polarization. Such materials as Polaroid enable us to demonstrate that light waves are transverse waves. A beam of light in which all the vibrations are in one direction is said to be linearly polarized, or plane polarized.