EIECTR031AGNETIC PHENOMENA. The connec tion between light and electromagnetic phenom ena is shown in several ways: the identity in the properties of and the waves pro duced by electric oscillations; the rotation of the plane of polarization in a magnetic field: the Kerr phenomenon; the Zeeman etTeet. etc.
It is shown in the article on ELECTRICITY that the waves produced by electric oscillations are transverse waves in the ether. and that they have the same velocity as They can be polarized, can he made to interfere, can be reflected, refracted, etc. The shortest wave length so far observed, for waves produced by electrical oscillations, is about two millimeters. It can be shown also that the velocity of ether waves in any transparent medium should vary as the square root of the reciprocal of the product of the electric and magnetic inductivities of that medium. The values of these quantities (K and /A ) for the pure ether can be written K, and ; therefore the index of refraction of the trans parent medium is n = For all such ciAo media K = isp; and K, = I on the C. G. S. trostatic system; so m = VK, where K is the value of the electric induetivity (or dielectric constant) of the transparent medium. This n, is the index of refraction for eery long Wares, because K is always measured by methods which involve slow oscillations. This formula is veri
fied by experiment in many cases.
As noted also in the article 011 ELECTRICITY, when a beam of plane polarized light is made to pass along the axis of a powerful eleetro. magnet, the plane of polarization is rotated in the direction in which the current in the mag netizing helix is flowing. This is called the Faraday effect. So, if the direction of the beam of light is reversed. the rotation is not. The amount of the rotation varies greatly with the transparent matter which occupies the magnetic field; it is proportional to the strength of the magnetic field; it varies with the wave-length and with the temperature.
It was stated in speaking of polarized light that if plane polarized light is incident upon a plate of polished metal, the reflected light is elliptically polarized. Kerr discovered that, if the metal surface is one end of a magnetized bar of iron, the reflected light is not the same as from a surface of unmagnetized iron. Ile found further that the changes were opposite in kind if the reflection took place from first a north pole and then a south one. Changes are also observed when the light is transmitted through thin films of magnetized iron.