Cotton-Mouton Effect


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Cotton-Mouton effect

[¦kät·ən ¦mü·ton i¦fekt]
(optics)
The double refraction (birefringence) of light in a liquid in a magnetic field at right angles to the direction of light propagation. Also known as Cotton-Mouton birefringence.

Cotton-Mouton Effect

 

double refraction of light in an isotropic substance placed in a transverse magnetic field (perpendicular to the light beam). It was first observed in colloidal solutions by J. Kerr and independently by the Italian physicist C. Maiorana in 1901. The French scientists A. Cotton and H. Mouton studied the effect in detail in 1907.

To observe the Cotton-Mouton effect, monochromatic light that is linearly polarized in a plane forming a 45° angle with the direction of the magnetic field is passed through a transparent isotropic specimen placed between the poles of a strong electromagnet. The material of the specimen becomes optically anisotropic in the magnetic field (its optical axis is parallel to the magnetic field H), and the light becomes elliptically polarized because it is propagating in the substance as two waves, the ordinary and extraordinary waves, which have different phase velocities. The difference in the indexes of refraction of the ordinary beam n0 and the extraordinary beam ne, called the double refraction value, is

ne — n0 = CH2 λ

where H is the magnetic field strength, C is a constant that depends on the substance and is called the Cotton-Mouton constant (see Table 1), and X is the wavelength of the light. The magnitude of C is inversely proportional to the absolute temperature T and is usually very small. Anomalously large values for C (10-8tolO-10) were observed in liquid crystals and in colloidal solutions. The value (ne — n0) has not yet been measured reliably in gases because the effect is very small.

Table 1. Values of the Cotton-Mouton constant for some
liquids at λ = 546 nm and T = 18°C
 C x 1013 cm1gauss-2
Benzene..............6.8–7.1
Toluene ..............6.7–7.3
Chlorobenzene ..............8.9
Nitrobenzene ..............25–30
Acetone ..............0.5

The Cotton-Mouton effect is related to a group of magnetic phenomena that also includes the Zeeman and Faraday effects. The theory of the Cotton-Mouton effect is analogous to that of the Kerr effect. A substance in a magnetic field becomes anisotropic as a result of ordered orientation of polarizable molecules or their aggregates in a magnetic field. The study of the Cotton-Mouton effect yields information on molecular structure, the formation of molecular aggregates, and molecular mobility.

REFERENCES

Born, M. Optika, Kharkov, 1937. (Translated from German.) Vol'kenshtein, M. V. Molekuliarnaia optika. Moscow-Leningrad, 1951.

V. A. ZAMKOV