REFRACTION, a deviation in the course of a ray of light when it passes through the surface of a transparent medium. A familiar result of refraction is seen when one looks very obliquely into a transparent pool of water, of which the bottom is visible. The water seems much shallower in such a case than it really is. Another example is the bent appearance of the oars of a boat when the blades are seen obliquely under water. When ordinary light is refracted, it is also decomposed into light of the various elementary colors. This decomposition is due to the unequal refraction of light of different wave-lengths; and the resulting separation is known as dispersion. It is too small to be noticed in ordinary cases, such as those we have already cited. But, on looking at a source of light through a glass prism, it is very evi dent. See LIGHT; SPECTRUM.
The general theory of refraction is quite simple, and may be expressed in the following way: The power of a transparent medium to refract a ray of light is expressed by a certain number called its index of refraction. The phenomenon of dispersion shows that this num ber is different for rays of different colors, or wave-lengths. It is generally greater the denser the medium; but to this rule there are many exceptions. The mathematical principle by which the amount of refraction is determined is as follows: Let AB represent the surface of a transparent medium, LM. Let a ray of light, PQ, strike this surface obliquely at Q and enter the medium, and let OR be its course through the medium. Draw through Q the perpendic ular DE. Then the angle PQD which the ray makes with the perpendicular before it enters the medium is called the angle of incidence; and the angle EOR which it makes after ing the medium is called the angle of refraction.
The law of refraction then is: For one and the same medium, and the same ray of light, the ratio of the sine of the angle of incidence to that of the angle of refraction is a constant quantity for the same medium and the same ray.
This constant ratio is called the index of re fraction. The law may be expressed in alge braic language as follows: Put I, the angle of incidence; R, the angle of refraction; n. the index of refraction.
Then we shall have sin I ts sin R or, sin I sin R It is a law of the course of light that, if a ray passes in the opposite direction, say from R to Q, it will continue on the line QP after leaving the medium. It follows if the ray PQ emerges from the medium at R it will suffer refraction according to the same law as a ray entering it, but in the reverse direction. Hence if the lower surface of the medium is parallel to the upper surface, the emerging ray RS will be parallel to PQ.
One result of the above law is that a ray entering or leaving a medium in a direction perpendicular to the surface of the latter will undergo no refraction, but will continue in the same straight line. In this case the angles of incidence and refraction are both zero.
From the above equation we find that, if the angle R and the index of refraction are given, we can find the angle of emergence by the equation sin I = nsinR.
Now, the angle R may be so large that this expression is greater than 1. Then there will be no possible value of the angle of emergence to fulfil the condition. It is found that, in this case, the ray will not emerge from the medium at all, but will be reflected from its inner surface as if the latter were polished and opaque. This is called total reflection, and is frequently applied in optical instruments. An example will show how this result may be brought about Let ABC, Fig. Z be the sec tion of a glass prism, the angle at C being a right angle. Let a ray enter the prism at the point Q in a direction perpendicular to the sur face BC. It will then pass on and will reach the lower surface of the prism at the point R.
the angle DRQ will then be 45°. The sine of this angle is 0.707+ Now, a glance at the table which follows will show that in all sorts of glass the index of re fraction is greater than 1.5. It follows that the sine of the angle at which the ray QR should emerge from the prism would be greater than 1.5 X 0.7, and therefore greater than 1. There is then no possible angle to fulfil the conditions. The light is therefore reflected in the direction RS, as though the surface AB were perfectly opaque and highly polished.