Free GCSE Physics lesson: Wave Behaviour
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Free Lessons -> GCSE / Key Stage 4 -> Physics -> Wave Behaviour

Lesson 17 · GCSE / Key Stage 4 · Physics

Wave behaviour and lenses

Explain reflection, refraction, diffraction and basic lens behaviour using wave ideas.

Qualification: GCSE Subject: Physics Waves

Waves

Lesson overview

This lesson introduces the core physics idea, the useful equipment and the calculation or data skills used on this page.

FocusReflection, refraction, diffraction and lenses
Time45-60 minutes
EquipmentRuler, ray diagram practice and calculator.
Practical linkNo separate practical method focus
Maths tagsM1 units and equation sense

What you will learn

  • Describe reflection using the normal line.
  • Explain refraction as a change in speed and direction.
  • Recognise diffraction through gaps and around obstacles.
  • Draw basic converging-lens ray behaviour.

Ray and ripple examples supplied on this page

Use the mirror, glass block, gap and lens examples to practise wave behaviour and diagram language.

Wave behaviour and lenses infographic

Infographic explaining GCSE Physics wave behaviour and lenses, including reflection, refraction, diffraction, absorption, convex lens ray diagrams, refraction rules, magnification and the wave speed equation.
Use this visual to connect wave behaviour, refraction rules, convex lens ray diagrams and equation checks.Download visual

Clear explanation

Reflection happens when a wave bounces from a boundary. The angle of incidence equals the angle of reflection, measured from the normal.

Refraction happens when a wave changes speed at a boundary, often changing direction. Light bends towards the normal when it slows down entering a denser medium.

Diffraction is spreading when waves pass through a gap or around an obstacle. It is strongest when the gap is similar in size to the wavelength.

Key diagram

Converging lens ray diagram with focal point Parallel rays enter a convex lens and converge through a focal point on the far side. focus parallel rays converge after passing through a convex lens
Diagram: the ray paths show why converging lenses focus parallel light and how focal length is identified.

Worked examples

Mirror reflection

A ray hits a mirror at 35 degrees to the normal.

Angle of incidence = angle of reflection.

The reflected ray is also 35 degrees to the normal.

Answer: The angle of reflection is 35 degrees.

Quick checks

Choose an answer, then check your thinking.

1. Angles of incidence and reflection are measured from which line?

2. When is diffraction strongest?

Practice questions

Question 1

A ray hits a plane mirror at 42 degrees to the normal. State the angle of reflection.

Reveal answer and marking guidance

Answer: 42 degrees.

Marking: Credit equal angles measured from the normal.

Question 2

Why does light refract entering glass from air?

Reveal answer and marking guidance

Answer: It changes speed at the boundary, so it changes direction.

Marking: Credit change in speed and direction.

Question 3

What happens to water waves passing through a narrow gap?

Reveal answer and marking guidance

Answer: They spread out by diffraction.

Marking: Credit spreading and link to gap size where possible.

Question 4

What does a converging lens do to parallel rays of light?

Reveal answer and marking guidance

Answer: It brings them together at a focus.

Marking: Credit convergence to the focal point.

Practice ladder

FluencyRecall the key definition, unit, equation or model before using the lesson questions.
ApplicationApply reflection, refraction, diffraction and lenses to an unfamiliar device, practical setup or data description.
Practical interpretationUse evidence, graph features, uncertainty, method quality or conclusion wording where the question asks you to evaluate.
Maths skillM1 units and equation sense

Answers and marking guidance

The exact practice answers are hidden under each question so you can try first. For this lesson, marks come from using the correct physics model, choosing the right equation where needed, keeping units with values, and explaining changes with precise words such as transfer, resultant force, acceleration, evidence and uncertainty.

Common mistakes

  • Measuring reflection angles from the surface instead of the normal.
  • Saying refraction is caused by frequency changing.
  • Forgetting diffraction can happen with any wave type.
  • Drawing lens rays that bend before they reach the lens.

Extension challenge

Draw a labelled ray diagram for a converging lens and describe how the focal length could be estimated.

Reveal answer

Example answer: A strong extension response names the physics model, uses accurate units and explains why the evidence supports the conclusion.

Exam-board guidance

Short board notes only. Learn the core physics above first.

AQA GCSE Physics

AQA GCSE Physics: often rewards clear physics explanations, correct equations, units and practical evidence for reflection, refraction, diffraction and lenses.

OCR GCSE Physics

OCR GCSE Physics: often values precise definitions, clear working, graph interpretation and links between models and evidence.

Pearson Edexcel GCSE Physics

Pearson Edexcel GCSE Physics: often combines the concept with equation use, data handling and practical interpretation.

Eduqas GCSE Physics

Eduqas GCSE Physics: learn the core explanation and practise applying it to unfamiliar contexts, data and practical questions.

WJEC Wales

WJEC Wales: often expects accurate terms, units and evidence-based explanations using the shared physics idea.

CCEA GCSE Physics

CCEA GCSE Physics: connect the idea to your current unit and use the same practical method language your class uses.

Next lesson

Next, continue with Nuclear Equations and Half-Life.

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