Application of Interference Methods 47
from a denser to a rarer medium, and consists in a bending of the incident ray toward the normal to the surface of the denser medium. Suppose we have a plate of glass, for example, and a ray of light falling upon the surface in any direction. According to the corpuscular theory, the substance below the surface exerts an attraction upon the light corpuscles. Such attraction can act only in the direction of the normal. If we separate it into two components, one in the surface and one normal to it, the normal one will be increased. These two components might be represented by OA and OB in Fig. 41, and the resultant of the two would be 00. In consequence of the presence of the denser medium, the normal component of the velocity of the particle is increased, and the resultant is now 00', which is greater than 00.
Let us next consider refraction according to the wave theory. A wave front ab (Fig. 42) is approaching the surface ac of a denser medium in the direction £c. This direction is changed by refraction to ce, and the corresponding direction of the new wave front is cd. During the time that the wave ab moves through the distance be in the rarer medium, it moves through the smaller distance ad in the denser. Thus the results, according to the two theories, are exactly reversed.
Hence, if we could measure the enormous speed of light — about 400,000 times as great as that of a rifle bullet—it would be possible to put the two theories to the test. In order to
48
Light Waves and Theib Uses
accomplish this we must compare the velocities of light in air and in some denser, transparent medium—say water. Now, the greatest length of a column of water which still permits enough light to pass to enable us to measure the very small quantities involved is something like thirty feet.
We should therefore have to determine the time it takes the light to pass through thirty feet of water, at the rate of 150,000 miles a second. This interval of time is of the order of one twenty-millionth of a second. But we must measure a time interval even smaller than this, for we have to distinguish between the velocity in water and the corresponding velocity in the air, /. r\, to determine the difference between two time intervals, each of which is of the order of one twenty-millionth of a second. This, at first sight, seems beyond the possibility of any physical experiment; but, notwithstanding this exceedingly small interval of time, by the combined genius of Wheatstone, Arago, Foucault, and Fizeau the problem has been successfully solved. The method proposed by Wheatstone for measuring the velocity of electricity was this: A mirror was mounted so that it could be revolved about an axis parallel to its surface at a very high rate, and the light from the spark produced by the discharge of a condenser was allowed to fall on the mirror. The images of two sparks were observed in the revolving mirror; the second spark passed after the electric current which produced it had passed through a considerable length of wire —
FIG. 42