Waves
This lesson builds reflection, refraction, diffraction and lenses for GCSE Physics.
Use the core lesson first, then match the exam-board guidance to your school route. Many pupils meet this content through Combined Science as well as Separate Physics.
What you will learn
Exam-board fit
Exact paper labels and specification-point numbering vary by board and cohort, so match this lesson to your school route before using past-paper questions.
Ray and ripple examples supplied on this page
Use the mirror, glass block, gap and lens examples to practise wave behaviour and diagram language.
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
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.
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.
Exam practice ladder
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.
Exam-board guidance
All supported routes assess the core physics idea, but they may group topics, practicals and paper wording differently.
AQA GCSE Physics
AQA GCSE Physics: use this lesson for reflection, refraction, diffraction and lenses, then check whether your class is taking Separate Physics or Combined Science.
OCR GCSE Physics
OCR GCSE Physics: the core physics idea is shared, but Gateway and Twenty First Century may organise questions differently.
Pearson Edexcel GCSE Physics
Pearson Edexcel GCSE Physics: practise the concept, the equation use and the practical language because questions often connect them.
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: check whether your class is using the current GCSE Physics route or a newer science route, then use this lesson for the shared physics idea.
CCEA GCSE Physics
CCEA GCSE Physics: connect the idea to your unit and remember that practical skills are assessed directly.
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.
Next lesson
Next, continue with Nuclear Equations and Half-Life.