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# Reflectors

## ray, refracted, surface, perpendicular and incident

REFLECTORS, SILVERING.—See Mirror, Silvering.

REFRACTION.—The bending or deflection of a ray of light (including heat and all other forms of radiant energy) which takes place whenever the ray passes at any other angle than a right angle from the surface of one medium into another medium of different density. This optical density by no means coincides with comparative specific gravity, e. g., turpentine is optically denser than water, but floats on the top of it. It is a retarding influence, and accordingly when the ray enters the denser medium at right angles, though not refracted, it is retarded in a certain proportion, traversing a less distance in a given time. Rays at other angles, it can be shown by analysis, must be bent aside according to a law discovered by Snell, about A.D. 162o. Let W W (fig. 384) represent the refracting surface of the denser medium—for example, water ; and draw A B perpendicular to that surface. Describe a circle round the point C where the per. pendicular cuts the surface. Now let a ray, D C, enter the surface at C, at some angle A C D with the perpendicular, and suppose it is found by experiment that the refracted ray takes the direction C d. In the first place the refracted ray will be found to be in the same plane as the incident ray. In the second place, if the medium below W W be the denser, the refracted ray will be bent towards the perpendicular, and the reverse in the contrary case, so that a ray, d c incident in water, would be refracted as C D, further away from the perpendicular on emergence into air. But thirdly, the refracted course of every other ray can now be calculated according to the following law. Draw D S and d s normal to the perpendicular, then the lines D S and d s

will represent, geometrically, the sines of the arcs A D and d B, and if the radius C A be unity, the numbers expressing S D and d s will be sines of the angles. The sine S D will have a cer tain ratio to the sine d s. And now if any other incident ray (E C) be taken, its sine found in the same way will be found to bear the same ratio to the sine of the refracted ray. This ratio of the sines is therefore invariable for all incidences for the same homogeneous substance. Such ratio is called its refractive index, and it will be readily seen how the index of any substance, as some kind of optical glass, being once found by simple experiment, the course of every refracted ray incident at any angle on the curved surface can be foreseen, and thus its focus or other properties calculated, or the curves calculated for a given focus, which is simply the point to which refracted rays converge.* Double Refraction.—Many crystals are not homogeneous, but have different properties of elasticity, etc., in different directions. The effect of such a constitution is that, unless a ray of light enters the crystal in some particular directions, it is not merely refracted in the manner described under refraction, but diverted into two rays. In this case the refracted ray or rays are not always in the same plane as the incident ray.

Plane of Refraction is the plane passing through the normal or perpendicular to the refract ing surface at the point of incidence and the refracted ray.