Refraction of Light

REFRACTION OF LIGHT, the change in the direction of a light ray when passing from one medium into another. The ratio of the sine of the angle of incidence to the sine of the angle of refraction, or, which is the same thing, the ratio of the propagation velocities of the light wave in one medium and in the other, is called the refractive index and is denoted by the letter n. If the first medium is a vacuum, then in this case the refractive index is called absolute. Usually, the refractive index n is taken as the index upon the transition of a light ray from air into a given medium, since the absolute refractive index of air for visible rays is very close to 1 (specifically, 1.0003). Upon the transition of a light ray from a medium with a small refractive index to a medium with a large refractive index, the ray approaches the normal, and vice versa. If a ray is incident at an angle greater than the so-called limiting angle (for glass and air it is equal to 45°), refraction does not occur, but the ray is entirely reflected from the boundary separating both media (so-called total internal reflection). This circumstance is widely utilized in a number of optical instruments, where prisms with total internal reflection successfully replace mirrors. When a ray falls on the surface of a prism at an angle less than the limiting one, the ray is refracted at both of its faces and deviates from the initial direction by a certain angle, depending both on the angle of inclination of one face to the other and on the wavelength. Monochromatic rays emerge from the prism in a parallel beam, while complex ones are scattered in a fan, with rays having shorter wavelengths (the violet part of the spectrum) being refracted more strongly (spectrum). This phenomenon takes place in the spectroscope, with the help of which in medicine one studies, for example, the absorption spectrum of blood and other colored liquids. In a more general case, when the boundary between two media is a spherical rather than a flat surface, a parallel beam of rays either converges at a single focal point (converging lenses) or diverges (diverging lenses). Different refraction for rays of different wavelengths is the cause of the so-called chromatic aberration, which is expressed in the appearance of colored contours around figures located in the field of view of the instrument. Chromatic aberration can be eliminated by a special selection of glasses. Almost all optical instruments used in medicine and biology are based on the refraction of light. The refractive index of light is usually determined by the refractometric methods of Abbe, Pulfrich, and others, based on the phenomenon of total internal reflection. The values of n for gases are as follows: air = 1; hydrogen = 0.473; oxygen = 0.924; nitrogen = 1.016. The value of n for a gas is influenced by temperature and the density of the latter. In some substances, a light ray after refraction travels not as a single beam, but as two, and their directions are at a certain angle to one another. Among such birefringent substances is the anisotropic substance of striated muscles.