Diffraction

OCR

GCSE

Diffraction is the spreading out of waves when they pass through a gap or travel around an obstacle. This phenomenon is most pronounced when the size of the gap or obstacle is comparable to the wavelength of the incident wave. While diffraction alters the direction of propagation and the geometry of wavefronts, it strictly preserves the wave's speed, frequency, and wavelength. It is a fundamental property of all wave types, including electromagnetic, sound, and water waves, and serves as a critical test for wave-like behavior.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

What You Need to Demonstrate

Key skills and knowledge for this topic

Award 1 mark for defining diffraction as the spreading out of waves when they pass through a gap or around an obstacle
Credit diagrams that show wavefronts curving at the edges when passing through a gap wider than the wavelength
Award 1 mark for explicitly stating that maximum diffraction occurs when the gap width is approximately equal to the wavelength
In diagrams, credit responses that maintain constant wavelength (spacing between wavefronts) before and after the gap
Award 1 mark for linking long-wavelength radio waves to their ability to diffract around large obstacles like hills

Example Examiner Feedback

Real feedback patterns examiners use when marking

"You have correctly identified the phenomenon as diffraction, but your diagram shows the wavelength changing — keep the spacing constant"
"Good definition, but to secure the marks for 'maximum diffraction', you must specify that the gap size equals the wavelength"
"You confused diffraction with refraction here; remember diffraction is 'spreading' at a gap, while refraction is 'bending' at a boundary due to speed change"
"Excellent application of physics principles to the radio wave context; ensure you explicitly mention the 'long wavelength' of the radio waves"

Marking Points

Key points examiners look for in your answers

Award 1 mark for defining diffraction as the spreading out of waves when they pass through a gap or around an obstacle
Credit diagrams that show wavefronts curving at the edges when passing through a gap wider than the wavelength
Award 1 mark for explicitly stating that maximum diffraction occurs when the gap width is approximately equal to the wavelength
In diagrams, credit responses that maintain constant wavelength (spacing between wavefronts) before and after the gap
Award 1 mark for linking long-wavelength radio waves to their ability to diffract around large obstacles like hills

Examiner Tips

Expert advice for maximising your marks

💡When drawing diffraction, use a ruler to ensure the spacing between wavefronts remains identical before and after the gap
💡Avoid the word 'bending' to describe diffraction; use 'spreading out' to clearly distinguish it from refraction
💡For Higher Tier application questions, explicitly compare the magnitude of the wavelength to the size of the obstacle to justify your answer

Common Mistakes

Pitfalls to avoid in your exam answers

Changing the wavelength (distance between lines) in diagrams after the wave passes through the gap
Confusing diffraction with refraction by describing the wave as 'bending due to change in speed' rather than 'spreading'
Drawing semi-circular wavefronts for gaps significantly larger than the wavelength (instead of just curving at edges)
Failing to relate the degree of diffraction to the relative size of the wavelength and the gap

Study Guide Available

Comprehensive revision notes & examples

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Key Terminology

Essential terms to know

Wavefront propagation and geometry

Relationship between gap width and wavelength

Conservation of wave parameters (v, f, λ)

Diffraction around obstacles versus through apertures

Likely Command Words

How questions on this topic are typically asked

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Draw

Describe

Explain

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Practical Links

Related required practicals

•{"code":"P5.2 Investigation","title":"Ripple Tank Experiments","relevance":"Observation of plane waves diffracting through varying gap sizes"}

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