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Physics · Waves
Refraction of waves
Describe refraction as a change of direction caused by a change of speed, construct ray diagrams for refraction, and (Higher tier) use wavefront diagrams (Required Practical 9, part 2).
Warm-up
Answer each one, then check.
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1
What is reflection?
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A wave bouncing off a surface
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2
What is a normal?
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A line at right angles to a surface
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3
Does light travel faster in glass or air?
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Air
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4
What is a medium?
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A material a wave travels through
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5
What is a wavefront?
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A line joining points of a wave in the same phase
Learning Objectives
- 1Describe refraction as a change in direction when a wave crosses a boundary.
- 2Explain refraction in terms of the change of speed (Higher tier).
- 3Construct ray diagrams for refraction at the boundary between two media.
- 4Use wavefront diagrams to explain refraction (Higher tier).
REFRACTION
Refraction is the change in direction of a wave when it passes from one medium to another, caused by a change in the speed of the wave.
Entering a denser medium (such as glass from air) the wave slows down and bends towards the normal. Leaving the denser medium it speeds up and bends away from the normal.
Refraction of Light
The ray emerges parallel to its original direction.
Required Practical 9: Refraction with a Glass Block
Trace the outline of the block and the rays before removing it.
Wavefronts and Refraction (Higher)
The change of speed makes the wavefront turn.
Direction of Bending
A ray of light passes from air into glass. Describe what happens to its speed and direction.
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- 1 Speed Decreases in the glass
- 2 Direction Bends towards the normal
AnswerIt slows down and bends towards the normal.
Explaining with Wavefronts (Higher)
Use wavefronts to explain why a wave changes direction when it enters a slower medium at an angle.
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- 1 The part of the wavefront that enters first Slows down first
- 2 The rest of the wavefront Is still travelling faster, so catches up
- 3 Result The wavefront turns, so the wave direction bends towards the normal
AnswerThe change of speed across the wavefront turns it.
Along the Normal
What happens to a ray that strikes the boundary at 0° to the normal?
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- 1 Angle of incidence 0°: travelling along the normal
- 2 Direction Does not change
- 3 Speed Still changes
AnswerIt is not refracted (no change in direction), but it still slows down.
Key Points
Exam-ready facts.
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Cause
A change of speed when the wave moves into a different medium.
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Frequency
Does not change.
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Different substances
EM waves can be absorbed, transmitted, refracted or reflected depending on wavelength (Higher).
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Practical
A ray box, glass block and protractor can be used to measure angles i and r.
Bend the Ray
Draw a ray diagram for light going from air into water at an angle, then from water into air. Label the normal and say whether the ray bends towards or away from it each time.
1. Draw the normal.
2. Decide if the speed decreases or increases.
A good answer shows: Air to water: slows down and bends towards the normal. Water to air: speeds up and bends away from the normal.
Can I...?
- 1Define refraction.
- 2Explain it with speed.
- 3Draw a refraction ray diagram.
- 4Say towards or away from the normal.
- 5Use wavefronts.
- 6Describe the practical.
- 7Say frequency is unchanged.
- 8Explain a ray along the normal.
Summary & Exam Focus
- Refraction is a change of direction due to a change of speed.
- Slows: towards the normal; speeds up: away.
- Wavefronts turn when the speed changes (Higher).
- Frequency stays the same.
Exam focus
Explain why a light ray bends towards the normal as it enters a glass block. (2 marks) (2 marks)
It slows down.
Key terms
The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.
- Refraction
- A change in direction of a wave caused by a change in speed.
- Normal
- A line at right angles to the boundary.
- Angle of refraction
- The angle between the refracted ray and the normal.
- Wavefront
- A line showing all points of a wave at the same stage.
- Denser medium
- A medium in which the wave travels more slowly.
- Boundary
- The surface between two media.
Practice questions
Have a go at each one before you open its answer.
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Question 1 State 2 marks
A ray of light passes from air into a glass block. Describe what happens to the speed of the light and the direction of the ray.
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Model answer
The speed decreases, and the ray bends towards the normal.
Mark scheme
- Speed decreases — 1 mark
- Bends towards the normal — 1 mark
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Question 2 Complete the diagram 3 marks
The diagram shows a ray of light hitting a glass block. Complete the diagram to show the path of the ray through the block and out of the other side.
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Model answer
The ray bends towards the normal entering the glass, then bends away from the normal leaving the block, emerging parallel to the incident ray.
Mark scheme
- Bends towards the normal inside the glass — 1 mark
- Bends away from the normal on leaving — 1 mark
- Emergent ray parallel to the incident ray — 1 mark
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Question 3 Explain 3 marks
Explain why a wave is refracted when it crosses the boundary between two media at an angle.
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Model answer
The speed of the wave changes when it enters the new medium. This changes the direction of the wave, because the wavefront slows down (or speeds up) first at one side.
Mark scheme
- Speed changes in the new medium — 1 mark
- Part of the wavefront changes speed first — 1 mark
- So the direction changes — 1 mark
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Question 4 Describe 5 marks
Describe an investigation to show how a ray of light is refracted by a glass block. Your answer should include the measurements you would take and what you would conclude.
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Model answer
Place a glass block on white paper and draw around it. Shine a narrow ray from a ray box at an angle to a face. Mark the incident ray, the emergent ray and draw the ray through the block by joining the points. Draw the normal and measure the angles of incidence and refraction with a protractor. Repeat for different angles. The ray bends towards the normal entering the glass.
Mark scheme
- Ray box, glass block on paper, outline drawn — 1 mark
- Incident and emergent rays marked — 1 mark
- Normal drawn, angles measured — 1 mark
- Repeat for different angles — 1 mark
- Conclusion: bends towards the normal in glass — 1 mark
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Question 5 Explain 2 marks
A wave travels from deep water to shallow water where it slows down. State what happens to the frequency and the wavelength.
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Model answer
The frequency stays the same. The wavelength decreases (v = fλ).
Mark scheme
- Frequency unchanged — 1 mark
- Wavelength decreases — 1 mark
Quick check
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Refraction is caused by a change in...
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A: speed
A change of speed.
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Light entering glass from air bends...
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B: towards the normal
It slows down.
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Leaving glass into air the ray bends...
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C: away from the normal
It speeds up.
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A ray along the normal is...
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D: not deflected
No change in direction.
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When a wave is refracted its frequency...
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A: stays the same
Set by the source.
Downloads
Free to keep, print and annotate.
- Refraction of waves.pptx Built from the lesson script on 30 September 2026. View
- Refraction of waves - Completed Notes.docx The full notes for the lesson, to revise from. Built from the lesson script on 30 September 2026. View
- Refraction of waves - Exam Questions.docx Built from the lesson script on 30 September 2026. View
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