Final answer:
Refraction occurs when a wave enters a new medium at an angle other than 90 degrees to the surface, causing it to bend due to a change in speed and wavelength based on the physical properties of the new medium.
Step-by-step explanation:
To have refraction occur as a wave enters a new medium, it must not enter at a right angle (90 degrees) to the surface. When a wave strikes the boundary between two media at an angle other than 90 degrees, it will bend or change direction. As it enters the new medium, its speed and wavelength will usually change due to the differing physical properties such as density or optical density between the two media. This is due to the wave's interaction with the medium at a molecular or atomic level, affecting its propagation speed.
This process can be visualized using Huygens's principle, which shows that each point on a wavefront can be considered as a source of wavelets spreading out in all directions. As these wavelets enter a medium where their speed is reduced, such as when light passes from air into water, they bend towards the perpendicular. This is described by Snell's law, which relates the angle of incidence and refraction to the indices of refraction of the two media.