It follows from the above that current instability in the case of solid dielectrics does not determine the electrical strength Ebr, but is a postbreakdown process.

LITERATURE CITED

i. G. I. Skanavi, Physics of Dielectrics (Strong-Field Region) [in Russian] (1958). 2. A. A. Vorob'ev and E. K. Zavadovskaya, Electrical Strength of Solid Dielectrics [in

Russian] (1956). 3o G. A. Vorob'ev, Physics of Dielectrics, Strong-Field Region (Summary of Lectures) [in

Russian] (1977). 4. G. A. Vorob'ev and I. S. Pikalova, Fiz. Tverd. Tela, 9, No. 4, 961 (1967). 5. G. A. Vorob'ev, N. I. Lebedeva, and G. S. Nadorova, Fiz. Tverd. Tela, 13, No. 3, 890

(1971). 6. G. A. Vorob'ev, L. S. Datsko, A. P. Druzhinin, S. G. Ekhanin, S. N. Morev, and N. S.

Nesmelov, Fiz. Tverd. Tela, 20, No. 4, 1059 (1978). Yu. N. Vershinin and Yu. A. Zotov, Fiz. Tverd. Tela, 17, No. 3, 826 (1975). Yu. N. Vershinin and Yu. Z. Zotov, Fiz. Tverd. Tela, 17, No. 12, 3487 (1976). A. B. Kunz, Phys. Rev., 175, No. 3, 1147 (1968). A. B. Kunz, Elementary Excitations in Solids, Molecules, and Atoms, Plenum Press (1974). G. A. Vorob'ev, Izv. Vyssh. Uchebn. Zaved., Fiz., No. 4, 127 (1974). G. A. Vorob'ev and V. A. Mukhachev, Breakdown of Dielectric Films [in Russian], Sov. Radio (1977). G. A. Vorob'ev and M. N. Lisetskaya, Zh. Tekh. Fiz., 36, No. i0, 1886 (1966). N. M. Torbin, in: Physics of Dielectrics [in Russian], Izd. Akad. Nauk SSSR (1960), p. 415.

.

8. 9.

i0. ii. 12.

13. 14.

POLARIZATION OF LIGHT BY REFLECTION

A. S. Lobarev UDC 53:371.3

In the "Optics" section of the general physics course for colleges (and also in text- books and laboratory courses) the question of polarization of light by reflection from a "vacuum--substance" interface is thoroughly analyzed [I, 2]. The case of complete polar- ization by reflection from a "matter--substance" interface is of no less interest.

It follows from the Fresnel formula [I, 2], that in the case of incidence of light on a "substance--vacuum" interface at the Brewster angle ~B the reflected ray is completely polarized, and

i ,~B = ~, (i)

where n is the absolute refractive index of the medium in which the refracted ray propagates,

The possibility of complete polarization by reflection from a "substance--vacuum" bound- ary can be established, and the correctness of Eq. (i) can be verified, on the apparatus shown schematically in Fig. i.

The system must be adjusted so that the axis of the light beam emerging from the colli- mator is perpendicular to the axis 0 (see Fig. i) and intersects it; in this case the light does not change its direction of propagation when it passes through the cylindrical surface, which greatly simplifies the determination of the angles of incidence on the plane surface in the investigation of reflection of light from the "substance --vacuum" boundary.

The observer examines the image of the collimator slit in the eyepiece; from the posi- tion of the eyepiece, lens 2 (lens 2 can rotate around the axis 0), and the coilimator it is easy to determine the angle of incidence. A rotating polaroid is used to determine when the ray is completely polarized. When the Brewster angle for incidence of light on the "sub-

V. I. Lenin Moscow Pedagogical Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 118-119, June, 1979. Original article submitted March 31, 1978.

0038-5697/79/2206- 0675507.50 �9 1979 Plenum Publishing Corporation 675

Fig. 1

f I .~ 7 - - �9

�9 ~ i v ' ~ s . , '

Fig. 2

Fig. i. i) Collimator with slit; 2) semicylindrical lens (0 is the cylinder axis); 3) polaroid; 4) eyepiece; 5) goniometer stage.

Fig. 2. Reflection and refraction of light on "sub- stance-- vacuum" boundary: I) substance; II) boundary layer; III) vacuum.

stance--vacuum" boundary has been determined the absolute refractive index of the substance can be determined from Eq. (i).

The same apparatus can be used to investigate polarization by reflection of light from a "vacuum--substance" boundary. To do this the lens is rotated so that the light emerging from the collimator falls on the plane face of the lens. The eyepiece must be replaced by a telescope, since in this experiment the parallel beam of light emerging from the collimator is still parallel after reflection from the plane face of lens 2.

In this experiment the Brewster angle, determined for reflection of light from a "glass--vacuum (air)" boundary, corresponds exactly with Eq. (i).

The complete polarization of the ray reflected from a "vacuum--substance" boundary was explained in [I] with the aid of a pictorial model, whose main ideas are used for the pro- posed model, which explains the complete polarization of a ray reflected from a "substance-- vacuum" boundary when the ray is incident on the interface at the Brewster angle.

The essence of the model is clear from Fig. 2, where E, R, and D are the amplitudes of the incident, reflected, and refracted waves, respectively. The subscript p denotes compo- nents parallel to the plane of incidence, and the subscript s denotes components perpendic- ular to the plane of incidence,

In the proposed mode it is assumed that:: I) the refracted ray is formed in a very thin boundary layer; 2) the reflected ray is formed by the same molecules of substance as the refracted ray; i.e., the refracted and reflected rays are secondary waves emitted by mole- cules of the substance in the boundary layer, which become dipoles owing to vibration of the electrons in a direction opposite to that of the electric vector of the light wave. Then, as Fig. 2 shows, when the light is incident at the Brewster angle Rp = 0 in the reflected ray, since Rp is due to vibrations of electrons in a direction coincident with the reflected ray (see Fig~ 2), but, as is known, a dipole does not emit in the direction of the axis.

LITERATURE CITED

i. G.S. Landsberg, Optics [in Russian] (19~6). 2. B.M. Yavorskii, Course of Physics [in Russian], Vol. 3 (1975).

6 7 6