A change in medium or a change in conditions may cause a wave to slow down. This slowing down has the effect of shortening the wavelength, although the frequency of the wave remains the same. This is shown for normal incidence in Fig. 1.13, which also shows what happens when the wavefronts meet the surface obliquely. One end of each wavefront, the end which touches the obstacle first, slows up before the other and that whole wavefront is made to swing round. The retardation of the wave causes deviation towards the normal at the surface. Conversely, if the waves travel faster in the second medium, they deviate away from the normal. This phenomenon is called refraction. It may occur sharply or it may occur gradually.