Waves do not always travel in straight lines past obstacles. They can spread into gaps and bend around edges. That spreading is diffraction, and it becomes most noticeable when the gap or obstacle size is similar to the wavelength.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
You can hear music around a doorway even when you cannot see the speaker directly. Why can sound spread around the opening so effectively, while visible light usually seems to travel straight through?
Type your prediction below. You will revisit it at the end.
Write your prediction in your book. You will revisit it at the end.
Wrong: Zero acceleration means an object is stationary.
Right: Zero acceleration means constant velocity — the object could be moving at constant speed in a straight line.
📚 Core Content
Diffraction is the spreading of waves around obstacles and through openings.
If waves pass through a very wide opening compared with their wavelength, they keep travelling in almost the same direction. If the opening becomes narrow enough relative to the wavelength, the outgoing wave spreads much more strongly. The same idea applies when waves encounter an obstacle edge — the wave bends into the shadow region behind the obstacle. This is not because the obstacle "pushes" the wave; it is a natural consequence of the wave nature of the disturbance.
Diffraction is most dramatic when the wavelength and the gap size are of the same order of magnitude. A water wave with wavelength 2 cm passing through a 2 cm gap will spread into a wide arc. The same wave passing through a 2 m gap will show almost no spreading at all. This dependence on relative size is one of the most important ideas in wave physics. It explains why some waves seem to travel in straight lines while others wrap around corners.
The most important comparison is not the absolute size of the gap, but the gap size relative to the wavelength.
When a wavefront reaches an opening, the edges of the opening act like new sources of circular or spherical wavelets — this is Huygens' principle in action. If the opening is very large, the edge effects are insignificant compared with the central part of the wave, so the beam continues almost straight. If the opening is small, the edge effects dominate, and the wavelets from the edges spread out across a wide angle. The transition from "straight through" to "strong spreading" happens gradually as the gap shrinks toward the wavelength.
| Gap compared to wavelength | What happens? | Example |
|---|---|---|
| Gap much larger than $\lambda$ | Little spreading, wave mostly continues straight | Light through a window |
| Gap about equal to $\lambda$ | Strong diffraction | Sound through a doorway |
| Gap smaller than or comparable to $\lambda$ | Very strong spreading | Water waves through a narrow slit |
Long wavelengths are more likely to be comparable to everyday obstacles and openings.
Sound waves and radio waves often have wavelengths large enough that doors, buildings, or hill edges are not enormous compared with them. Visible light has a tiny wavelength (around 500 nm), so doors and walls are huge compared with the wavelength, which is why its diffraction is less obvious in everyday situations. However, scientists can create extremely narrow slits — much smaller than a human hair — to observe light diffraction clearly. This was one of the key pieces of evidence that established light as a wave.
The relationship is not that long wavelengths "like" to diffract more; it is simply that our everyday environment is built on a scale of metres and centimetres, which happens to match the wavelengths of sound and radio waves. If humans were the size of bacteria, we would notice light diffracting around specks of dust just as readily as we notice sound diffracting around doorways.
Can spread through a doorway and around corners more easily than visible light.
Long wavelengths can diffract around structures more noticeably than high-frequency light.
Does diffract, but the effect is usually subtle because the wavelength is so short.
Ripple tanks show diffraction clearly because we can vary the gap and watch the wavefront shape change.
A broad opening lets nearly straight wavefronts continue through. A narrow opening produces almost circular wavefronts beyond the gap. This visual evidence helps us connect the abstract rule "gap comparable to wavelength" with a clear physical observation. In the laboratory, students can place barriers with different gap widths in a ripple tank and directly observe how the spreading changes. When the gap is wide, the shadow behind the barrier is well-defined. When the gap is narrow, there is almost no shadow — the waves fill the space behind the barrier.
Ripple tanks are also excellent for demonstrating that diffraction occurs for obstacles as well as openings. A single post in a ripple tank casts a V-shaped diffraction pattern behind it. The wavelength, the obstacle size, and the angle of spreading can all be measured directly from the projected image. This hands-on evidence makes diffraction one of the most accessible wave phenomena to study experimentally.
Diffraction is not limited to openings — waves also bend around the edges of obstacles.
When a wave encounters a solid obstacle, the edge of the obstacle behaves like a new point source of wavelets. These wavelets spread into the region that would otherwise be in shadow. The amount of spreading depends on the same ratio: obstacle size compared to wavelength. A large island will cast a sharp acoustic shadow for sound waves, but a small post will not. This is why you can hear someone talking on the other side of a thin tree trunk but not on the other side of a warehouse.
Obstacle diffraction has important engineering consequences. Architects designing concert halls must account for how sound diffracts around balcony edges and columns. If the features are too large compared with sound wavelengths, they create unwanted acoustic shadows where the music sounds muffled. If the features are small, diffraction smooths the sound distribution across the audience.
A few common ideas about diffraction are wrong — and examiners know them well.
✏️ Worked Examples
Scenario: Water waves of wavelength 2 cm pass through two gaps. Gap A is 20 cm wide. Gap B is 2.5 cm wide. Which gap shows stronger diffraction?
If the wavelength were made much shorter while the gap stayed 2.5 cm, the diffraction would become less noticeable.
Scenario: Explain why you can hear someone speaking around a doorway more easily than you can see them if they are fully out of sight.
If the opening were extremely tiny, even visible light could show noticeable diffraction patterns.
Scenario: A radio wave of wavelength 150 m encounters a hill that is 50 m wide. A second radio wave of wavelength 2 m encounters the same hill. Compare the diffraction of the two waves around the hill.
If the hill were 200 m wide, even the 150 m wave would show weaker diffraction, though still more noticeable than the 2 m wave.
Visual Break
🏃 Activities
For each case, state whether diffraction would be weak, moderate, or strong:
Write two or three sentences explaining why sound diffracts around a doorway more noticeably than visible light.
A student says, "Only water waves diffract." Correct this statement with one clear sentence.
An AM radio station broadcasts at 1000 kHz (wavelength $\approx$ 300 m). An FM station broadcasts at 100 MHz (wavelength $\approx$ 3 m).
You have a ripple tank, a set of barriers with different gap widths (1 cm, 5 cm, 10 cm, 20 cm), and a wave source that produces waves with wavelength 2 cm.
Earlier you were asked why you can hear music around a doorway more easily than you can see the speaker directly.
The full answer: sound has a much longer wavelength than visible light, so a doorway is more comparable in size to the wavelength of sound. That makes diffraction more noticeable for sound. Visible light also diffracts, but far less obviously in an everyday doorway because its wavelength is tiny.
Now revisit your prediction. What role does wavelength play in your explanation?
Annotate your prediction in your book with what you now understand differently.
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
✅ Check Your Understanding
1. Diffraction is best described as:
2. Diffraction is greatest when:
3. Which wave usually shows more noticeable diffraction around everyday openings?
4. A gap is 4 cm wide and the wavelength is 4 cm. The diffraction will be:
5. Which statement about diffraction is correct?
6. A wave with a longer wavelength will usually diffract more noticeably because:
7. Explain why diffraction becomes strong when the gap size is similar to the wavelength. 3 MARKS
8. A water wave has wavelength 3 cm. Compare the expected diffraction for gaps of 30 cm and 4 cm. 3 MARKS
9. Explain why radio waves are often more noticeable than visible light in diffraction around buildings or obstacles. 4 MARKS
1. B — diffraction is the spreading of waves around obstacles and through openings.
2. C — diffraction is strongest when gap size is comparable to wavelength.
3. D — sound usually shows more noticeable diffraction around everyday openings.
4. A — a gap matching the wavelength gives strong diffraction.
5. B — all wave types can diffract.
6. C — long wavelengths are more likely to match everyday obstacle sizes.
Q7 (3 marks): Diffraction becomes strong when the gap size is similar to the wavelength because the opening is no longer extremely large compared with the wavefront spacing. In that case, the edges of the gap act as significant sources of spreading wavelets according to Huygens' principle. The wave emerging from the gap spreads out strongly instead of continuing almost straight ahead. The closer the gap is to the wavelength, the more noticeable the spreading becomes.
Q8 (3 marks): With wavelength 3 cm, a 30 cm gap is ten times larger than the wavelength, so diffraction will be weak and the wave will continue mostly straight with only slight spreading at the edges. A 4 cm gap is only slightly larger than the wavelength, so diffraction will be much stronger and the wave will spread out clearly into a wide arc after passing through the opening.
Q9 (4 marks): Radio waves often have much longer wavelengths than visible light. Because of that, buildings, doorways, and obstacle edges are more likely to be comparable to radio wavelengths than to light wavelengths. This makes diffraction more noticeable for radio waves. Visible light also diffracts, but its wavelength is so small that ordinary obstacles are enormous by comparison, so the spreading is usually much less obvious and requires specialised equipment to detect.
Activity 2: Sound diffracts around a doorway more noticeably than visible light because the wavelength of sound is comparable to the width of a doorway, whereas the wavelength of visible light is many orders of magnitude smaller. Since diffraction is strongest when the gap size is similar to the wavelength, sound bends significantly while light travels almost straight through.
Activity 4: (1) AM radio has a much longer wavelength (≈ 300 m) than FM radio (≈ 3 m). Hills and valleys are comparable to AM wavelengths, so AM signals diffract around terrain features more effectively. FM signals have wavelengths much smaller than typical hills, so they tend to be blocked. (2) FM radio uses higher frequencies, which can carry more information and are less susceptible to certain types of electrical interference, providing clearer, higher-fidelity sound. (3) The AM signal will diffract more around the 20 m building because its wavelength (300 m) is much larger than the building, whereas the FM wavelength (3 m) is much smaller than the building.
Activity 5: (1) The 1 cm gap would produce the strongest diffraction because it is closest to the wavelength of 2 cm. (2) With the 20 cm gap, the wave would show very weak diffraction and continue almost straight through with nearly straight wavefronts. (3) A gap much smaller than the wavelength would be difficult because viscous forces and surface tension would dominate, distorting the wave and making it hard to produce clean wavefronts.
Scale the platforms using your knowledge of wave diffraction and interference patterns. Pool: lessons 1–6.
Tick when you have finished the activities and checked the answers.