Reflection and Refraction

This lesson covers the behaviour of waves when they meet a boundary — specifically reflection and refraction — as required by the Edexcel GCSE Physics specification (1PH0), Topic 4: Waves. You need to understand the law of reflection, be able to draw ray diagrams for reflection and refraction, and explain why waves change direction when they enter a different medium.

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Reflection of Waves

Reflection occurs when a wave bounces off a surface and changes direction. All types of waves can be reflected — including light, sound, and water waves.

The Law of Reflection

The law of reflection states:

The angle of incidence equals the angle of reflection.

i = r

Where:

• i = angle of incidence (the angle between the incoming ray and the normal)
• r = angle of reflection (the angle between the reflected ray and the normal)

The Normal Line

The normal is an imaginary line drawn perpendicular (at 90°) to the reflecting surface at the point where the incident ray hits. All angles are measured between the ray and the normal — not between the ray and the surface.

Drawing a Reflection Ray Diagram

1. Draw the reflecting surface (e.g. a mirror) as a straight line.
2. Draw the normal as a dashed line, perpendicular to the surface at the point of incidence.
3. Draw the incident ray arriving at the surface.
4. Measure the angle of incidence between the incident ray and the normal.
5. Draw the reflected ray on the other side of the normal at the same angle as the angle of incidence.
6. Mark the angles of incidence (i) and reflection (r) clearly.
7. Add arrows on the rays to show the direction of travel.

Exam Tip: Always measure angles from the normal, not from the surface. If the angle between the ray and the surface is 30°, then the angle of incidence is 90° − 30° = 60°. This is a common exam mistake.

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Refraction of Waves

Refraction is the change in direction of a wave when it passes from one medium to another. Refraction occurs because the wave changes speed when it enters a different medium.

Why Does Refraction Occur?

• When a wave passes from one medium to another (e.g. from air into glass), its speed changes.
• If the wave hits the boundary at an angle (not along the normal), the change in speed causes the wave to change direction.
• The frequency of the wave stays the same — it is the wavelength and speed that change.

Rules for Refraction

Scenario	Speed	Direction	Wavelength
Wave enters a denser medium (e.g. air → glass)	Decreases	Bends towards the normal	Decreases
Wave enters a less dense medium (e.g. glass → air)	Increases	Bends away from the normal	Increases
Wave hits the boundary along the normal (angle of incidence = 0°)	Changes	No change in direction	Changes

Key Points

• Refraction only causes a change in direction if the wave hits the boundary at an angle.
• If the wave hits the boundary head-on (perpendicular / along the normal), it slows down (or speeds up) but does not change direction.
• The frequency never changes during refraction — this is because the number of waves entering the new medium per second must equal the number leaving.

Exam Tip: In refraction questions, always state three things: (1) the wave speed changes, (2) the wave direction changes (bends towards or away from the normal), and (3) the wavelength changes. But the frequency stays the same. This is a very commonly tested point.

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Refraction of Light Through a Glass Block

This is a standard experiment and a common exam diagram:

What Happens

1. A ray of light travels through air and hits a rectangular glass block at an angle.
2. At the first boundary (air → glass): the light slows down and bends towards the normal (because glass is denser than air).
3. Inside the glass block, the light travels in a straight line (no further boundaries).
4. At the second boundary (glass → air): the light speeds up and bends away from the normal (because air is less dense than glass).
5. The emergent ray is parallel to the incident ray but displaced (shifted sideways).

flowchart LR
    A["Incident ray<br/>(in air)"] --> B["First boundary<br/>Air → Glass<br/>Bends TOWARDS normal<br/>Speed decreases"]
    B --> C["Ray travels through<br/>glass block<br/>(straight line)"]
    C --> D["Second boundary<br/>Glass → Air<br/>Bends AWAY from normal<br/>Speed increases"]
    D --> E["Emergent ray<br/>(in air, parallel to<br/>incident ray but displaced)"]

    style A fill:#3498db,color:#fff
    style B fill:#e67e22,color:#fff
    style C fill:#95a5a6,color:#fff
    style D fill:#e67e22,color:#fff
    style E fill:#3498db,color:#fff

Drawing a Refraction Ray Diagram

1. Draw the glass block as a rectangle.
2. Draw the normal at each boundary (perpendicular to the surface).
3. Draw the incident ray arriving at the first boundary at an angle.
4. At the first boundary (air → glass): draw the refracted ray bending towards the normal.
5. Continue the ray through the block to the second boundary.
6. At the second boundary (glass → air): draw the refracted ray bending away from the normal.
7. Label all angles of incidence and refraction.
8. Add arrows showing the direction of light.

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Why Waves Change Speed in Different Media

The speed of a wave depends on the properties of the medium it is travelling through:

• Light travels fastest in a vacuum (3 × 10⁸ m/s) and slower in denser materials like glass or water.
• Sound travels fastest in solids (particles close together) and slowest in gases (particles far apart).

Medium	Speed of Light (approx.)	Speed of Sound (approx.)
Vacuum	3 × 10⁸ m/s	Cannot travel
Air	~3 × 10⁸ m/s	~330 m/s
Water	~2.3 × 10⁸ m/s	~1500 m/s
Glass	~2 × 10⁸ m/s	~4500 m/s
Steel	—	~5900 m/s