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incident beam of polarized light contains all sorts of wave lengths, as in white light, the crystal plate may appear colored when seen through the analyzer. For, suppose one component of the long waves of red light is retarded a half wave length behind the other in traversing the crystal plate, the emergent light will be vibrating at right angles to the incident beam, as shown in figure 607, and will be wholly transmitted by the analyzer.

But in traversing the same crystal plate the shorter waves of violet light may have one component retarded a whole wave length behind the other; in this case the relation of phase is the same in the emergent as in the entering beam, and the beam coming from the crystal is suppressed by the analyzer.

For some intermediate wave length the relative retardation will be of a wave length, and the emergent beam will be circularly polarized and so half transmitted by the analyzer.

On the whole, therefore, the crystal in this case would appear red or orange through the analyzer. But from a crystal plate of twice the thickness both the red and violet waves would emerge vibrating as in the incident beam and would be suppressed by the analyzer, while some intermediate wave length would be completely transmitted and the crystal would appear green.

963. Polarization Figures with Convergent Light.-When a thin plate of crystal is examined in an instrument called a polariscope,

Polarizer

E

Analyzer

G'

FIG. 608.-Optical system of polariscope.

using a strongly convergent beam of polarized light, a polarization figure is obtained which is of great use to the mineralogist in revealing the optical properties of the crystal.

The optical system of the polariscope is shown in figure 608. The beam of light coming from the polarizer is converged on the crystal by the lens B. On the opposite side of the crystal plate C' is a second shortfocus lens D, beyond which is the eye lens E and analyzer A.

If the crystal is a plate cut from a uniaxial crystal perpendicular to its axis and if the analyzer and polarizer are crossed, a figure consisting of colored rings intersected by a black cross, as shown in figure 609, is seen at F'O'G' by the observer.

It is clear that rays coming to O in the center of the figure are those that have passed perpendicularly through the crystal section, while rays coming to other points of the figure have passed more or less obliquely through the crystal. Now, the more oblique the rays the greater the thickness of crystal traversed, hence the color seen at any point in the figure depends on the distance of that point from the center at 0.

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FIG. 609.-Polarization figure for uniaxial crystal, FIG. 610.-Polarization figure perpendicular to axis, Nicols crossed.

of biaxial crystal.

All points on a ring equidistant from O show the same color, because the rays at these points have traversed equal thickness of crystal at an equal inclination to the optic axis.

Rays passing through the crystal in certain directions, however, have their vibrations in such relation to the optic axis of the crystal that they are transmitted without any change in their polarization. All such are cut out by the analyzer and form the black cross.

In figure 610 is shown a more complicated polarization figure. produced by a biaxial crystal, such as mica.

Reference on Polarization.

Edwin Edser. Light for Students.

ELECTRICITY AND LIGHT.

964. Magnetic Rotation of Light.-When a transparent substance is in a powerful magnetic field a beam of plane polarized light sent through it in the direction of the lines of force,

has its plane of polarization rotated. This discovery was made by Faraday in 1845 and was the first evidence of a relation between light and electricity and magnetism.

The rotation is usually in the direction of the magnetizing current, or clockwise looking in the direction of the lines of force, though it is opposite in a solution of ferric chloride in water. The amount of the rotation is greatest in substances having a large index of refraction, and in a given substance is proportional to the length of the column and to the strength of the magnetic field.

If the light is reflected back again through the tube the rotation is doubled; that is, the rotation of the plane of polarization produced in this way is the same whether the light passes through the field in the positive direction of the lines of force or the

reverse.

This last fact can be explained only by supposing an actual rotatory motion of some sort taking place in the magnetic field.

In this respect the magnetic rotation of the plane of polarization is different from that produced by quartz.

965. The Kerr Effect. Closely related to the rotation discovered by Faraday is the fact, discovered by Kerr, that when a beam of polarized light is reflected from the polished pole of a magnet the plane of polarization is rotated.

966. Maxwell's Electromagnetic Theory of Light.-In the year 1862 Maxwell advanced the theory that light waves are very short electromagnetic waves. Some of the chief arguments for the theory may be thus summarized:

1. The velocity of electromagnetic waves in air is the same as that of light waves. The velocity of a wave depends on the medium in which the disturbance is set up and the kind of disturbance. It is therefore reasonable to suppose that electric waves are the same kind of disturbance as light waves and communicated by the same medium-the luminiferous ether.

2. The velocity of light in other media than air has in many cases been found to be equal to the velocity of electric waves in those media.

3. Maxwell showed that electromagnetic waves could not pass through conductors, hence it was to be expected that conductors would also be opaque to light. This is strikingly confirmed in the

case of metals, for they are the best conductors of electricity and also the most opaque substances known.

4. The electric currents and displacements in the medium transmitting electric waves are parallel to the wave front and at right angles to the direction in which the wave is advancing; and in light waves also the vibrations are parallel to the wave front.

967. Zeeman Effect.-A very interesting relation between electricity and light which seems to offer direct confirmation of Maxwell's theory that light waves are electromagnetic was discovered by Zeeman, of Holland, in 1896. He found that when a sodium flame or other luminous gas giving out light of definite wave lengths, as shown by lines in its spectrum, is placed in the powerful magnetic field between the poles of an electromagnet, each single line in its ordinary spectrum is transformed into a group of lines. The peculiarities of these groups have been explained by the Dutch physicist, Lorentz of Leyden, on the assumption that light waves are electromagnetic waves originating in little vibrating negatively charged electrons having the same mass and charge as the electrons in cathode

rays.

For a fuller discussion of the Zeeman effect see Modern Theory of Physical Phenomena by Righi.

968. Pressure of Light.-It was shown by Maxwell in 1873 that if fight waves are electromagnetic they must exert a pressure against any surface on which they fall; and that the pressure against a reflecting surface must be twice as great as against an absorbing one. But the amount of this force is so small that for many years no one succeeded in proving its existence; for in full sunlight, according to Maxwell's theory, the pressure against a reflecting mirror one meter square is only one dyne, or less than the weight of one milligram.

But in 1900 Lebedew in Russia, and in 1901 Nichols and Hull in this country, were able to show that there is such a pressure, and in later experiments to prove that its amount is just what Maxwell's theory indicates.

The pressure of light has been shown by Fitzgerald and Arrhenius to be the probable cause of comet's tails, and Arrhenius has also proposed a very interesting explanation of the Aurora Borealis which depends in part on this same pressure.

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