What's the difference between diffraction and interference?

Diffraction is the bending of waves around obstacles, while interference is the combination of two or more overlapping waves.

Diffraction is a wave phenomenon that occurs when a wave encounters an obstacle or a slit. It is characterised by the bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The bending, into the region of shadow, occurs significantly when the size of the obstacle or aperture is comparable to the wavelength of the wave. This phenomenon is observed in light, sound, and water waves. For example, when light passes through a narrow slit and is viewed on a screen, a series of light and dark bands are observed. This is due to diffraction of light.

Interference, on the other hand, is a property of waves that enables them to superpose, or add together, resulting in a new wave. When two or more waves meet, they interact with each other. If the waves are in phase (i.e., their peaks and troughs line up), they will interfere constructively, resulting in a wave with a larger amplitude. If they are out of phase (i.e., the peak of one wave lines up with the trough of another), they will interfere destructively, resulting in a wave with a smaller amplitude or even no wave at all. This phenomenon is also observed in light, sound, and water waves. For example, in a double-slit experiment with light, an interference pattern of light and dark bands is observed on a screen due to the constructive and destructive interference of light waves.

In summary, while both diffraction and interference are fundamental wave phenomena, they describe different aspects of wave behaviour. Diffraction describes how waves bend around obstacles or through apertures, while interference describes how waves combine when they overlap. Both phenomena are crucial in understanding the wave nature of light, sound, and other types of waves.