4.3a Wavefronts, rays and graphs

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Wavefronts

We can examine waves in one dimension (as along a string or a slinky), two dimensions (such as waves on the surface of water) or three dimensions (such as light or sound).

A wavefront is a way of describing and visualising the propagation of waves. This is probably easiest to do for two dimensional waves, such as water waves in a pool. If you can picture all the points on that surface that are in phase together, such as the peaks or crests of the waves (or the dips or troughs) and join them up on a diagram you have the wavefronts. If there is a continuous travelling wave then there will be a series of wavefronts, each separated from the other by a distance of one wavelength. This is easy to picture for water waves, but the same applies to light, sound, etc.

Rays

Sometimes you just want to examine a wave along its direction of propagation. A light bulb may fill a room with light but you are only interested in the light that leaves the bulb, reflects from a surface and enters your eye. We can describe this small bit of a wave, and the path it takes, as a ray. All waves, including light, water and sound, can be described as rays. We can reproduce approximate rays experimentally for light using ray-boxes or lasers. Note that rays are ALWAYS perpendicular to wavefronts.