Text-book of general physics for high schools and colleges

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1904 Excerpt: ... the wave A C at the instant when the region C has reached the point B. For there is no other line below the plane AB, which, like BN, is a common tangent to all these secondary waves.... "If, now, using the same figure, we draw EAF normal to the plane AB at the point A, and draw DA at right angles to the wave AC, the incident ray of light will then be represented by DA; and AN, which is drawn perpendicular to BN, will be the refracted ray; for these rays are merely the straight lines along which the parts of the waves travel. "From the foregoing, it is easy to deduce the principal law of refraction; viz., that the sine of the angle DAE always bears a constant ratio to the sine of the angle NAF, whatever may be the direction of the incident ray, and that the ratio is the same as that which the speed of the waves in the medium on the side AE bears to their speed on the side AF. "For, if we consider AB as the radius of a circle, the sine of the angle BAC is BC, and the sine of the angle ABN is AN. But the angles BAC and DAE are equal, for each is the complement of CAE. And the angle ABN is equal to NAF, since each is the complement of BAN. Hence the sine of the angle DAE is to the sine NAF as BC is to AN. But the ratio of BC to-4iVis the same as that of the speeds of light in the media on the side toward AE and the side toward AF, respectively; hence, also, the sine of the angle DAE bears to the sine of the angle NAF the same ratio as these two speeds of light." Since these speeds are properties of the media and not of the direction of the propagation of the waves, we have at once the law that the ratio of these sines is independent of the angle of incidence. It is evidently different for different media, and will be shown to be different ...