Physics · Wave behaviour. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A wavefront joins points that are in phase; a ray shows the direction of travel and is at right angles to the wavefronts. At a boundary a wave is partly reflected and partly transmitted. If its speed changes it also changes direction, unless it travels along the normal. This is refraction. The frequency never changes, so the wavelength changes with the speed.
Diffraction is the spreading of a wave through a gap or round an obstacle. It is greatest when the gap is about the same size as the wavelength.
When two waves meet, their displacements add. This is superposition. Coherent sources give a steady pattern: the waves reinforce where they arrive in phase and cancel where they arrive exactly out of phase.
Students measure angles from the surface instead of from the normal, or say the frequency changes on refraction. Angles are always from the normal, and only the speed and the wavelength change.
Answer: About 62°.
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