For IB Physics SL and HL: A wavefront joins points on a wave that are in the same phase, while a ray shows the direction in which the wave travels. Rays are perpendicular to wavefronts in an isotropic medium.
This distinction belongs to C.3 Wave phenomena and is used when representing reflection, refraction and diffraction.
| Feature | Wavefront | Ray |
|---|---|---|
| Meaning | A line or surface joining points with the same phase | A line showing the direction of wave propagation |
| Direction | Extends across the wave | Points in the direction of travel |
| Spacing | Adjacent equivalent wavefronts are separated by one wavelength, | Spacing between rays does not represent wavelength |
| Typical shape | Straight for plane waves; curved for circular or spherical waves | Straight in a uniform medium; changes direction at a boundary |
| Relationship | Perpendicular to the ray | Perpendicular to the wavefront |
For example, parallel straight wavefronts represent a plane wave. Rays drawn at right angles to these wavefronts are also parallel and show the wave's direction of travel.
For a point source, the wavefronts are concentric circles in two dimensions or spheres in three dimensions. The rays point radially outward, remaining perpendicular to each wavefront.
During refraction, the wave speed changes at a boundary. The frequency remains constant, so means the wavelength and wavefront spacing change. If one side of a wavefront enters the new medium first, it changes speed first, causing the wavefront and its perpendicular ray to change direction.
A common misconception is that a ray is the path of an individual particle. It is not: a ray is a geometrical representation of the direction of energy propagation.
Exam technique: If asked to draw or annotate a wavefront-ray diagram, show rays perpendicular to the wavefronts, include an arrow on each ray, and measure reflection or refraction angles from the normal; not from the boundary.