GCSE Physics resources

AQA GCSE Waves

Reflection, Refraction and Diffraction

Reflection sends a wave back. Refraction occurs when speed changes in a new medium. Diffraction is strongest when the gap size is similar to the wavelength and spreading increases around narrower gaps.

AQA 8463 GCSE Physics FoundationHigher

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The revision snapshot

Reflection sends a wave back. Refraction occurs when speed changes in a new medium. Diffraction is strongest when the gap size is similar to the wavelength and spreading increases around narrower gaps.

By the end, you should be able to:

  • Explain what changes at a boundary.
  • Apply the waves model, evidence or method to an unfamiliar exam context.
  • Recognise and correct this wrong turn: Measuring from the surface.

Before you revise

Diagnostic question

Choose an answer from memory. Your result tells you what to watch for in the guide.

Which statement correctly summarises reflection, refraction and diffraction?

Core revision guide

Learn the model, then use it

Reflection, Refraction and Diffraction questions become manageable when the central model, evidence and exam method are kept together. Read the explanation, test the misconception and then apply the idea without looking back.

Explain what changes at a boundary

Reflection sends a wave back. Refraction occurs when speed changes in a new medium. Diffraction is strongest when the gap size is similar to the wavelength and spreading increases around narrower gaps.

Build the physics picture

At a reflecting boundary, the wave returns to the original medium. Refraction happens because speed changes at a boundary, changing wavelength and often direction. Diffraction is spreading after a gap or edge and becomes most obvious when the gap is similar in size to the wavelength.

The relationship for this guide

Wave speed: v = fλ (m/s, Hz, m). Choose each relationship from the physical change described, convert quantities into compatible units and keep the unit beside the final answer.

Three panels show equal reflection angles, refraction towards the normal in a slower medium and diffraction through a wavelength-sized gap.
Measure angles from the normal and link greater diffraction to a gap similar in size to the wavelength.Open the full-size exam diagram
Wave speedv = fλm/s, Hz, m

Misconception clinic

Replace the tempting answer

Read each belief, say what is wrong with it, then compare your correction.

Tempting idea: Measuring from the surface.

Use instead: Reflection sends a wave back. Refraction occurs when speed changes in a new medium. Diffraction is strongest when the gap size is similar to the wavelength and spreading increases around narrower gaps.

Tempting idea: Calling any bend reflection.

Use instead: At a reflecting boundary, the wave returns to the original medium. Refraction happens because speed changes at a boundary, changing wavelength and often direction. Diffraction is spreading after a gap or edge and becomes most obvious when the gap is similar in size to the wavelength.

Tempting idea: Saying diffraction only happens at a gap.

Use instead: Draw the normal where needed, label angles from the normal and explain the path using speed, wavelength and boundary conditions.

Retrieval practice

Quick checks

1. Which exam method is most reliable for reflection, refraction and diffraction?

2. Which statement correctly applies reflection, refraction and diffraction to a question?

3. A pupil writes: “Measuring from the surface.” Which replacement is accurate?

Explore the relationship

Wave boundary explorer

Compare the gap width with wavelength to predict how strongly waves diffract.

What to notice: use the readout to describe how one variable changes when the other is controlled.

Worked examples

See the method being built

These examples expose the difference between a secure explain what changes at a boundary method and the common error “Measuring from the surface.”.

Example 1 · Reflection law

A ray meets a mirror at 35° to the normal. Find reflection angle.

  1. Angles are measured from normal.
  2. Use angle i = angle r.
  3. Give degrees.
Show the answer

Reflection angle = 35°.

Example 2 · Refraction

A light ray enters glass from air and bends towards the normal. Explain.

  1. Identify new medium.
  2. State speed decreases.
  3. Link to change in direction.
Show the answer

Light slows in glass, so it refracts towards the normal.

Example 3 · Diffraction

Compare a 1 cm gap and 10 cm gap for 2 cm waves.

  1. Compare gap with wavelength.
  2. Identify stronger spreading.
  3. State conclusion.
Show the answer

Diffraction is strongest at the 1 cm gap because its size is similar to the 2 cm wavelength.

Exam precision

Exam technique: Explain what changes at a boundary

Do this: Draw the normal where needed, label angles from the normal and explain the path using speed, wavelength and boundary conditions.

Independent practice

Exam-style questions

Write an answer before opening the marking guidance. Then edit the exact phrase or step that would gain the next mark.

1. State the law of reflection.

[1 mark]
Show marking guidance and model answer

Marking guidance: Award one mark for the precise statement shown in the model answer.

Model answer: Angle of incidence equals angle of reflection.

2. State what happens to wave speed during refraction.

[1 mark]
Show marking guidance and model answer

Marking guidance: Award one mark for the precise statement shown in the model answer.

Model answer: It changes on entering a different medium.

3. State when diffraction is most noticeable.

[1 mark]
Show marking guidance and model answer

Marking guidance: Award one mark for the precise statement shown in the model answer.

Model answer: When gap size is similar to wavelength.

4. Why draw a normal?

[2 marks]
Show marking guidance and model answer

Marking guidance: 2 marks are available for relevant, linked physics points that match the model answer.

Model answer: Angles of incidence, reflection and refraction are measured from it.

5. A ray hits a mirror at 50° to the normal. Find reflected angle.

[3 marks]
Show marking guidance and model answer

Marking guidance: Award one mark for the correct relationship, one for a valid substitution and one for the final answer with its unit.

Model answer: 50°.

6. Explain why a wave does not change frequency on refraction.

[3 marks]
Show marking guidance and model answer

Marking guidance: Award up to 3 marks for distinct physics points joined into a clear cause-and-effect chain.

Model answer: The source still produces the same number of waves per second; speed and wavelength change instead.

Questions pupils ask

Reflection, Refraction and Diffraction FAQs

What is the main idea in Reflection, Refraction and Diffraction?

Reflection sends a wave back. Refraction occurs when speed changes in a new medium. Diffraction is strongest when the gap size is similar to the wavelength and spreading increases around narrower gaps.

What mistake should I avoid in Reflection, Refraction and Diffraction?

Measuring from the surface. At a reflecting boundary, the wave returns to the original medium. Refraction happens because speed changes at a boundary, changing wavelength and often direction. Diffraction is spreading after a gap or edge and becomes most obvious when the gap is similar in size to the wavelength.

Useful next steps

Connect this guide to your next task

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