When a light ray traveling through air or a vacuum enters a denser medium, it reduces its speed by index of refraction. Different media (which are transparent to light) have different indices of refraction. For example, the speed of light in air is 300,000 km/s and almost the same as in vacuum. If a light ray enters glass, for example, which has an index of 1.5, the speed is reduced to 200,000 km/s.
According to the wave theory of light, the reduction of the light speed is reflected in its shortened wavelength. This phenomenon represents the base of the concept of refraction. If a light ray enters the glass perpendicularly, the wavelength of the light ray shortens, but when the ray exits the glass it resumes to normal speed, that is, returns to the original “air wavelength” and continues its travel in the same direction. If, however, the light ray enters the glass at any angle other than the perpendicular, interesting things happen: the light ray (considered to be of a wave nature in this case) has a front that does not enter the glass media at the same time because it comes under an angle. The parts of the front that enter the glass first are “slowed down” first.
The end result is the refraction of the light ray; the ray does not continue in the same direction but deflects slightly. This deviation depends on the density of the media.
The denser the media – that is, the higher the index of refraction – the greater the inclination of the original direction.
There is a very simple relation between the angles of incidence and refraction and indices of refraction between the two different media. This relation was discovered by the Dutch physicist Willebrord Snell in the early seventeenth century. By using a very simple calculation, we can determine the angles of refraction in various media. As we shall see later on, the same concepts are used when calculating the angles of total reflection and numerical aperture in fiber optics.
The basics of refraction are graphically explained in the diagram on the previous page, where it is assumed a monochromatic (single frequency) light ray enters the glass. The bottom drawing also shows that a percentage of the incident light is always reflected back into air (or vacuum); in the case of glass this percentage is very small.