Describe diffraction as the spreading of waves around obstacles and through openings, and explain interference as the superposition of waves, distinguishing constructive and destructive interference and standing waves.

What this topic is asking

This dot point covers two wave behaviors that are signatures of waves: diffraction (spreading around obstacles and through gaps) and interference (the combining of overlapping waves). The Physical Setting/Physics course asks you to describe diffraction, to explain interference through the principle of superposition, to distinguish constructive from destructive interference, and to recognize standing waves with their nodes and antinodes. Crucially, the interference of light is evidence that light is a wave.

Diffraction

Diffraction is why you can hear someone around a corner (sound waves spread around the edge) and why a wave passing through a narrow harbour mouth fans out beyond it. For light, with its very short wavelength, diffraction is noticeable only through very narrow slits or around tiny obstacles, which is why we usually see light travelling in straight lines. When the gap is much larger than the wavelength, the wave passes almost straight through with little spreading.

Interference and superposition

Interference produces patterns of reinforcement and cancellation. Where two sources are in phase, points equidistant from both (or differing by a whole number of wavelengths) interfere constructively, and points differing by half a wavelength interfere destructively. This creates the alternating bright and dark bands seen when light passes through two slits, a pattern that only waves can produce, which is why interference is taken as proof of the wave nature of light.

Standing waves

A standing wave forms when two identical waves travel in opposite directions and superpose, for example a wave reflecting back on itself along a string or in a pipe. The result appears to stand still: certain points, the nodes, never move (destructive interference always), while points midway between them, the antinodes, oscillate with maximum amplitude (constructive interference). Standing waves explain the resonant notes of musical instruments, where only certain wavelengths fit the length of the string or air column.

Reference Tables note

There is no equation for diffraction or interference on the Physical Setting/Physics Reference Tables (no double-slit or diffraction-grating formula), so this topic is assessed qualitatively. You recall that diffraction is wave spreading (greatest when the gap is near the wavelength), that interference follows superposition (constructive in phase, destructive out of phase), and that standing waves have nodes and antinodes. The wave nature of light shown here contrasts with its particle nature in the dual nature of light.

Try this

Q1. State what happens to the amplitude when two waves interfere constructively. [1 point]

• Cue. The amplitude increases (the displacements add because the waves are in phase).

Q2. State what a node is on a standing wave. [1 point]

• Cue. A point that never moves (always destructive interference), of zero amplitude.