Waves can be represented in two complementary ways: wavefronts and rays.
Wavefronts
A wavefront is a line joining points on a wave that are in phase (e.g. all at the crest). In a ripple tank, wavefronts appear as lines of crests.
- For a point source, wavefronts are circular (spreading outward).
- Far from the source, wavefronts become approximately straight (plane waves).
- The spacing between wavefronts equals the wavelength.
Rays
A ray is an arrow drawn perpendicular to the wavefronts, showing the direction of energy transfer.
- Rays are always at right angles to wavefronts.
- Ray diagrams are used for showing reflection, refraction and the paths of light through lenses and prisms.
Using wavefronts to show wave behaviour
Reflection: Wavefronts bounce off a barrier. The angle of incidence equals the angle of reflection.
Refraction: Wavefronts change direction as they cross a boundary where the wave speed changes. The side that enters the new medium first slows down (or speeds up), causing the wavefront to rotate.
Diffraction: Wavefronts spread out after passing through a gap or around an obstacle. Maximum diffraction occurs when the gap width is similar to the wavelength.
Common errors and how to correct them
- Drawing rays parallel to wavefronts instead of perpendicular.
- Forgetting that wavefronts are equally spaced when wave speed is constant.
How to apply this in an exam
Draw wavefronts as parallel lines with equal spacing. Add rays perpendicular to the wavefronts. Show the change in spacing or direction when the wave enters a different medium.
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