Diffraction is the spreading of a wave as it passes through a gap or round an obstacle. The effect is greatest when the gap width is similar to or smaller than the wavelength. All waves diffract — sound, light, water, and radio — though the effect is most obvious with long wavelengths.
What is wave diffraction?
Diffraction occurs whenever a wave passes through a narrow gap or past the edge of a barrier. Instead of continuing in a perfectly straight line, the wave spreads out into the region beyond the gap. This is a property of all waves — it cannot happen to a stream of particles — and so observing diffraction is itself evidence that something is behaving as a wave.
Diffraction does not change the wave's speed, wavelength, or frequency. Only the direction of energy spread changes. The wave still travels at the same speed in the same medium; it just fans out rather than propagating in a single direction.
What conditions produce the most diffraction?
The key rule is:
Diffraction is greatest when the gap width is equal to or smaller than the wavelength of the wave.
As the gap narrows relative to wavelength:
- When gap width >> wavelength: very little spreading; most energy continues straight through
- When gap width ≈ wavelength: maximum spreading; the wave fans out in a broad semicircle
- When gap width < wavelength: the gap acts as a point source; circular wavefronts emerge
This relationship explains an important everyday observation: sound (wavelength ~centimetres to metres) diffracts noticeably around corners and through doorways, while visible light (wavelength ~400–700 nm) does not spread around a doorframe in any obvious way.
How does wavelength affect the amount of diffraction?
For a fixed gap width:
- Longer wavelengths diffract more — they spread into a wider angle
- Shorter wavelengths diffract less — they continue more nearly straight ahead
This means red light (λ ≈ 700 nm) diffracts slightly more than violet light (λ ≈ 400 nm) through the same gap. For radio waves, long-wave radio (λ ≈ 1 km to 10 km) diffracts easily around hills and buildings, making it receivable in valleys where FM radio (λ ≈ 3 m) cannot reach — FM shadows entire valleys behind terrain.
| Wave type | Typical wavelength | Noticeable diffraction through a 1 m gap? |
|---|---|---|
| Long-wave radio | ~1 km | Yes — far exceeds gap width |
| FM radio | ~3 m | Moderate |
| Sound (middle A) | ~0.77 m | Yes — similar to many gaps |
| Visible light | 400–700 nm | Essentially none |
| X-rays | 0.01–10 nm | Only through atomic-scale gaps |
Why does light not obviously diffract through doorways?
The wavelength of visible light is roughly 500 nm (5 × 10⁻⁷ m). A typical doorway is about 1 m wide — that is two million times larger than the wavelength. With such a vast mismatch, the spreading angle is immeasurably small to the naked eye. Light does diffract through a doorway in principle, but the spreading angle is so tiny (~0.00003°) that it appears to travel perfectly straight.
To observe light diffraction easily, you need a gap comparable to its wavelength — a fine diffraction grating or a single thin slit. A CD or DVD makes a visible diffraction pattern in white light because the track spacing (~1.6 μm) is comparable to the wavelength of visible light.
What are real-world examples of wave diffraction?
- Long-wave radio broadcasting: wavelengths of 1–10 km diffract around the entire curvature of the Earth, allowing BBC Radio 4 Longwave to be received thousands of kilometres from the transmitter without line-of-sight.
- Harbour waves: water waves entering through a narrow harbour entrance spread out behind it, reaching boats moored at the sides of the harbour — not just those directly in line with the entrance.
- Acoustic design: concert hall designers account for sound diffraction around balcony edges to ensure even distribution to all seats.
- Electron diffraction: electrons accelerated through a high voltage have wavelengths similar to the spacing of atoms in a crystal lattice. When fired at a thin graphite film, they produce a diffraction pattern — direct evidence that particles have wave properties (wave-particle duality).
How is diffraction shown in wave diagrams?
In a ripple tank diagram, incoming plane waves (straight parallel lines) that pass through a narrow gap emerge as curved wavefronts, with the curvature most pronounced at the edges. For a very narrow gap (width ≈ wavelength), the pattern after the gap looks almost like the circular waves produced by dropping a stone in a pond.
Key features to include when drawing a diffraction diagram:
- Straight, evenly-spaced incoming wavefronts approaching the gap
- The gap in the barrier, with width comparable to the wavelength spacing shown
- Curved outgoing wavefronts beyond the gap, spreading sideways
- Spacing between outgoing wavefronts equal to the spacing of the incoming wavefronts (wavelength unchanged)
Frequently asked questions
What is diffraction in GCSE physics?
Diffraction is the spreading of a wave when it passes through a gap or around the edge of an obstacle. The amount of spreading depends on how the gap width compares with the wavelength: maximum diffraction occurs when gap width equals wavelength, and very little occurs when the gap is much wider than the wavelength. All waves can diffract — this property distinguishes waves from particles.
Why does sound diffract more easily than light?
Sound waves have wavelengths ranging from a few centimetres (high-pitched sounds) to several metres (low-pitched sounds), similar in scale to everyday gaps like doorways and windows. Light has wavelengths of only ~400–700 nm — far smaller than any gap we encounter in daily life. Because diffraction is greatest when gap width matches wavelength, sound diffracts noticeably around everyday objects while light appears to travel in perfectly straight lines unless the gap is engineered to match its tiny wavelength.
Does diffraction change the speed or wavelength of a wave?
No. Diffraction changes only the direction in which the wave spreads. The speed, wavelength, and frequency remain identical on both sides of the gap. This distinguishes diffraction from refraction, in which the wave crosses into a different medium and its speed and wavelength change (though frequency stays constant). An exam question asking whether speed or wavelength changes during diffraction should always receive the answer: neither changes.
How is diffraction evidence that light is a wave?
If light were purely a stream of particles, it would travel in straight lines through a gap with no spreading. The fact that it produces a diffraction pattern — bright and dark fringes when passed through a very narrow single slit or diffraction grating — can only be explained if light has wave properties that allow it to interfere with itself. Young's double-slit experiment (1801) first demonstrated light diffraction and interference, providing convincing evidence that light is a wave.
For Socratic GCSE physics with Professor Newton — predict whether sound or light will spread more through a 1 m gap before drawing any wavefront diagram, then check whether the wavelength comparison explains your prediction — visit aitutors.me.