A Physic’s definition of refraction involves a change in the direction of waves as they pass from one medium to another. This is due to energy conservation when a wave changes its wavelength, and thus wave speed, as it enters a new medium (like in light or sound waves traveling from air to water). Now, in terms of ocean waves, refraction also involves a change of direction when a wave experiences a shift in wavelength and wave speed. However, this shift generally occurs due to changes in ocean depth. As a wave travels from deep to shallow water, the wavelength shortens, the wave speed slows down, and the wave will refract, or bend, toward the shallow area in order to conserve its energy. This is commonly seen in deep water canyons where the wave encounters shallow water along the canyon’s edges, and the wave will bend/refract towards the shallow water. Sean Collin’s explains how this can benefit surfers in the previous blog “Waves at Newport Pier?”
Diffraction occurs when a wave encounters an obstruction in its path and will change direction, or wrap around it. In ocean waves, we see this occur when a wave encounters an object like a jetty and the wave rotates around it (sometimes diffraction also occurs when a waves moves through a small opening in a seawall or between or two islands). The ‘wrapping’ or turning potential of a wave is larger in waves with a longer wavelength (i.e. longer period). This is why a long period Groundswell wave can sometime wrap a full 180 degrees around a barrier/jetty, whereas short-period Windswell wave will often shoot straight by it. Diffraction can occur in shallow or deep water and is separate from refraction since it is not a result of a change in ocean depth. However, both refraction and diffraction will involve a change in a wave’s direction.
Now, you may see these two used interchangeably in ocean waves because we often see swell encountering an Island and both refraction and diffraction will occur in this case. The Island acts as a barrier and we see diffraction occur as the waves wrap around it. The bending of the waves then intensifies as they encounter shallower water around the Island, thus refraction occurs.
Hope this helps clear up the difference,
The Surfline forecast Team