wavefront

A wavefront is a surface that connects all the points of a wave that are in the same part of their cycle — all at a crest, say, or all at a trough. For ripples spreading on a pond the wavefronts are the expanding circles; for light from a distant star they are nearly flat sheets. Drawing wavefronts is a clean way to picture how a wave is shaped and which way it is heading, since the direction of travel is always at right angles to the front.

Wavefronts make the behaviour of waves easy to visualise. When a wave passes through a narrow opening, the wavefronts curl around the edges and spread out, which is diffraction. When two sets of wavefronts overlap, the places where crest-lines cross crest-lines are where the wave is strongest, tracing out the bright fringes of an interference pattern. Much of optics can be reasoned out just by sketching how wavefronts bend, spread, and overlap.

In quantum mechanics the same picture applies to the wavefunction of a particle: surfaces of equal phase are the matter wave's wavefronts, and they propagate, bend around obstacles, and interfere just as light's do. Thinking in wavefronts is a helpful bridge for seeing why a particle described by a wave can diffract through a slit or build an interference pattern, even though it will ultimately be detected at a single point.