Explain diffraction and gap-width effects
Describe wave spreading at an aperture and predict how the gap-to-wavelength ratio affects diffraction.
Section 8 · Superposition
AS Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 8. These original models and calculations are study aids; full timed assessment, supervised practical competence and human subject-teacher review remain outstanding.
Before you begin
- Use wavelength, frequency, phase difference, trigonometry and consistent SI units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens when a wave diffracts at an aperture?
What you will learn
- Define diffraction as wave spreading at an aperture or obstacle
- Describe a ripple-tank investigation of diffraction
- Relate diffraction strength qualitatively to gap width compared with wavelength
Diffraction is the spreading of waves
Diffraction is the spreading of a wave as it passes through a gap or around an obstacle. A narrow aperture can act as a source of spreading wavefronts on the far side.
The amount of spreading depends on the aperture width relative to the wavelength. Diffraction is most noticeable when the gap width is similar to the wavelength; a much wider gap gives less spreading relative to the forward direction.
Compare gap sizes in a controlled ripple-tank test
In a ripple tank, generate water waves with a fixed frequency and hence a fixed wavelength in the same depth. Observe their fronts before and after a gap, then vary only the gap width and compare the spreading pattern.
Keep wave frequency, water depth and viewing scale constant so the wavelength does not change while gap width is compared. Record whether fronts spread widely or remain mainly forward. This is a qualitative comparison; the syllabus does not require a numerical diffraction formula here.
Pause and explain
When is aperture diffraction most noticeable?
Worked example
Water waves of wavelength 2.0 cm pass through a 2.0 cm gap, then through a 10 cm gap. Which gap produces more noticeable diffraction?
Show the worked solution
- Compare each aperture width with the same wavelength.
- The 2.0 cm gap is comparable to the 2.0 cm wavelength.
- The 10 cm gap is much wider than the wavelength.
Answer The 2.0 cm gap produces more noticeable spreading.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A ripple tank uses 3 cm wavelength waves. Compare expected spreading through gaps of 3 cm and 15 cm.
Give me a hint
Compare each gap width with the wavelength, keeping the water and source unchanged.
Compare my reasoning
- The first gap is equal in scale to λ.
- The second gap is five times λ.
- Expect more noticeable spreading at the comparable-width gap.
The 3 cm gap gives more noticeable diffraction; the 15 cm gap is much wider than the wavelength.
Look for these in your work
- Both widths compared with λ
- Qualitative trend applied
- Same-wave condition retained
Describe a fair ripple-tank experiment to test how aperture width affects diffraction.
Give me a hint
Change the aperture width while controlling the wave pattern and tank conditions.
Compare my reasoning
- Generate water waves at a fixed frequency in uniform depth.
- Observe the spreading beyond one gap width, then alter only the gap.
- Compare the wavefront curvature or lateral spreading for each width.
Keep source frequency, water depth and observation scale fixed; vary gap width and compare the spreading of the outgoing wavefronts.
Look for these in your work
- Independent variable identified
- Relevant controls stated
- Observable comparison stated
A student claims a 12 cm gap produces no diffraction because the waves mainly travel forward. Explain why that conclusion is too strong when λ = 2 cm.
Give me a hint
Distinguish weaker relative spreading from zero spreading.
Compare my reasoning
- The gap is six times the wavelength, so spreading is less noticeable than at a comparable gap.
- Diffraction still occurs at an aperture.
- The correct conclusion is mainly forward propagation with less relative spreading, not no diffraction.
The gap is much wider than λ, so diffraction is less pronounced, but the wave still spreads at the aperture; it is not correct to say diffraction is absent.
Look for these in your work
- Gap-to-wavelength ratio assessed
- Diffraction definition retained
- Strength distinguished from absence
Common mistakes
- Claiming a wave only diffracts when gap width equals wavelength exactly
- Changing frequency and gap width together in a comparison
- Confusing diffraction with reflection at the gap
What can you explain now?
What happens when a wave diffracts at an aperture?
Compare with the explanation
It spreads as it passes through the gap
Diffraction describes wave spreading at an aperture or obstacle.
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Make it stick
Why should frequency and water depth stay fixed when testing the effect of gap width?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.