On the reflection and decomposition of light at the separating sur­faces of media of the same and of different refractive powers

Abstract

When white light is incident upon a surface which separates two different media, the portion that is reflected should, according to the Newtonian theory of light, preserve its whiteness, provided the thick­ness of either of the media exceed the eighty millionth of an inch. But since the dispersive powers of bodies are different, it must follow as a necessary consequence, that reflected light can never under any circumstances retain perfect whiteness, although the modification it experiences is not of sufficient amount to become sensible in ordinary experiments. The author during his investigations of the laws of polarization for light reflected at the separating surface of different media, had occasion to inclose oil of cassia between two prisms of flint glass, and was surprised to find that the light reflected was cf a blue colour. The fact was new, but might be readily explained upon the principle that although the refractive density of oil of cassia greatly exceeds that of flint glass for the mean rays, yet the action of these two bodies is nearly the same on the less refrangible rays: hence it may happen that a larger proportion of the former than of the latter is transmitted, and the pencil formed by reflexion will then appear blue. The partial decomposition thus effected in the incident rays will be the same in kind, though it may vary in degree, at different angles of incidence, and cannot therefore give rise to any variation of colour in the reflected rays, although they may differ in intensity according to the obliquity of the incidence. By using dif­ferent kinds of glass, and of interposed fluids, the author obtained various analogous results, different rays of the spectrum being sepa­rated according to the prevalence, in each particular case, of one or other of the opposite actions exerted upon them by the solid and the fluid medium. The author directed his attention more particularly to those conditions in which the nearest approach could be made to a perfect equilibrium of all the forces which affect the incident rays. The solids which he employed in his experiments were two prisms of plate glass, of which the sections were right-angled isosceles triangles, and differing hut very slightly in their refractive indices. The fluids were castor oil and balsam of copivi, the former having a less, and the latter a greater refractive power than the glass prisms; a thin film of either fluid being interposed between them. With castor oil, and within the limit of total reflexion, the reflected light is yellow; on gradually diminishing the angle of incidence, it passes in succession through all the tints of three orders of colours, of which the details are presented in a table exhibiting those which correspond to different angles of incidence. When the incident light is homogeneous, no colours are seen, but the reflected pencils have their maxima and minima of intensity; like the rays of thin plates, or the fringes of inflected light formed by homogeneous rays. When copivi balsam is employed as the fluid medium, the same orders of colours are ob­tained by reflexion, but at smaller angles of incidence than with cas­tor oil. Having ascertained that at a temperature of about 94° the mean refractive index of the balsam became equal to that of the glass prisms, the author examined the influence of a gradual elevation of temperature upon the colours of the reflected pencils ; and found that no particular change marked the instant when the refractive densi­ties of the two media became equal; although when the temperature was increased considerably, the tints entirely disappeared. Analogous results were obtained by employing prisms of obsidian instead of glass.