Light passes more slowly through a substance than through a vacuum. As light enters a denser substance, the advancing waves at the interface are modified by being closer together owing to their slower speed and shorter wavelength, as shown in Figure 4-16. If the light enters the denser substance at an angle, as shown, one part of the wave slows down more quickly as it passes the interface, and this produces a bending of the wave toward the interface. This phenomenon is called refraction. If light enters a less dense substance, it is refracted away from the interface rather than toward it. The relative value of this effect between two substances is given by the refractive index, n:
= (4-23)