Final answer:
A change in the speed of a wave as it enters a new medium causes refraction, which alters the wave's direction. This is due to the differing densities of the mediums involved, as demonstrated by Huygens's principle and Snell's law.
Step-by-step explanation:
A change in the speed of a wave as it enters a new medium produces a change in the wave's direction, a process known as refraction. This phenomenon occurs because different media have different properties, such as density, that affect the wave's speed. For example, when light moves from air into water, it slows down and bends towards the normal due to water's higher density. The same principle applies to water waves going from deep to shallow water; they slow down and their wavelength decreases as they enter the shallower water, bending the path of the wave closer to the perpendicular. This bending of the wave is explained by Huygens's principle of wavefronts and can lead to the derivation of Snell's law for calculating the angle of refraction.