When waves travel across a boundary between two different media, their speed changes. For example, the speed of light in a vacuum is 3×10⁸ms^-1, whereas in glass its speed is about 2×10⁸ms^-1. The change in speed can result in a change in direction of propagation of the waves. This change in direction is called refraction.

The diagrams below show how to use Huygens’ principle to predict the position of the wave-front when waves move from a medium in which they have speed v₁, to a medium in which they have speed v₂. In this case, v₂ < v₁. In these diagrams the line x x’ represents the boundary between the two media.

At time t = 0, point A on the wave-front reaches the boundary. Consider "secondary wavelets" emitted from A at time t = 0. At time t seconds later, point B reaches the boundary. At time t, the "secondary wavelets" emitted from A have moved a distance v₂t.

The position of the new wave-front is shown by line C D.

The situation at a later time is shown in the next diagram. Notice that the change in speed of the waves inevitably produces a change in the wavelength.