Wavefronts & Rays

Before we can talk about waves bending, bouncing and overlapping, we need two simple ways of drawing them. Wavefronts capture the shape of a wave; rays capture the direction it’s heading. Get these straight and every later diagram falls into place.

Waves in Two and Three Dimensions

Drop a stone in a pond and the ripples spread out as growing circles — that’s a surface wave, travelling in two dimensions with circular wavefronts. Sound and light instead spread out in every direction through space as growing spheres — spherical wavefronts in three dimensions. (A spherical wavefront of radius r spreads its energy over a sphere’s surface area, A = 4πr², which matters later when we look at intensity.)

Wavefronts

A wavefront is a line joining all the points of a wave that are oscillating in phase — all at a crest together, say, or all at a trough together. Because those points move in step, the wavefront marks the “leading edge” of one part of the wave. Each wavefront is one wavelength behind the last.

Rays

A ray is simply an arrow showing the direction the wave is travelling (and carrying its energy). The key rule to remember: a ray is always drawn perpendicular — at 90° — to the wavefronts. Viewed from above, a flat wave’s wavefronts look like a series of parallel straight lines, and the ray cuts straight across them.

Reading Wavefront Diagrams

In exam diagrams, crest wavefronts are usually drawn as darker solid lines and troughs as fainter or dashed lines — though some diagrams show only the crests. Whichever is used, the rule for measuring is the same: the distance between two successive crest wavefronts (or two successive trough wavefronts) is exactly one wavelength.