Reflection and Refraction of Waves

This lesson covers the reflection and refraction of waves — two key behaviours that all waves exhibit — as required by AQA GCSE Combined Science Trilogy (8464), Physics Paper 2, section 6.1. You need to understand what happens when waves meet a boundary and be able to draw accurate ray diagrams.

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Reflection

Reflection occurs when a wave bounces off a boundary (a surface) and changes direction.

The Law of Reflection

$\text{angle of incidence} = \text{angle of reflection}$angle of incidence=angle of reflection

$\theta_i = \theta_r$θi​=θr​

graph TD
    N["Normal (perpendicular to surface)"]
    I["Incident ray"] -->|"θi"| P["Point of incidence on surface"]
    P -->|"θr"| R["Reflected ray"]

    style N fill:#7f8c8d,color:#fff
    style I fill:#2980b9,color:#fff
    style P fill:#2c3e50,color:#fff
    style R fill:#e74c3c,color:#fff

Key Definitions

• Normal — an imaginary line drawn at right angles (90°) to the surface at the point where the ray hits.
• Angle of incidence (θi) — the angle between the incident ray and the normal.
• Angle of reflection (θr) — the angle between the reflected ray and the normal.

Exam Tip: Angles are always measured from the normal, not from the surface. This is a very common mistake. If the question says the angle of incidence is 30°, the ray makes a 30° angle with the normal line (and therefore a 60° angle with the surface).

Reflection of Waves in a Ripple Tank

When straight wavefronts hit a flat barrier in a ripple tank, they reflect so that the angle of incidence equals the angle of reflection. The wavelength and speed of the waves do not change upon reflection.

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Refraction

Refraction is the change in direction of a wave when it passes from one medium to another, caused by a change in speed.

Why Does Refraction Happen?

When a wave crosses a boundary between two media at an angle:

• One side of the wavefront enters the new medium first and changes speed.
• The other side is still in the original medium, travelling at the original speed.
• This difference in speed causes the wavefront to change direction.

Key Rules for Refraction

Scenario	Speed	Direction
Wave enters a denser medium (e.g. air → glass)	Slows down	Bends towards the normal
Wave enters a less dense medium (e.g. glass → air)	Speeds up	Bends away from the normal
Wave hits the boundary along the normal (0°)	Changes speed	No change in direction

graph LR
    subgraph "Refraction: Air → Glass"
    direction LR
    A["Air (less dense)"] -->|"Incident ray"| B["Boundary"]
    B -->|"Refracted ray bends towards normal"| C["Glass (denser)"]
    end

What Stays the Same During Refraction?

• The frequency of the wave does not change when it is refracted.
• The speed and wavelength change.
• Since $v = f\lambda$v=fλ and $f$f is constant, when speed decreases the wavelength also decreases (and vice versa).

Property	Changes during refraction?
Speed	Yes — changes at the boundary
Wavelength	Yes — changes with speed
Frequency	No — stays the same
Direction	Yes — unless the wave hits along the normal

Exam Tip (AQA 8464): A very common exam question: "What happens to the frequency when a wave is refracted?" The answer is always: the frequency stays the same. Only the speed and wavelength change.

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Refraction of Water Waves in a Ripple Tank

You can demonstrate refraction in a ripple tank by placing a glass plate under the water to create a shallow region:

• In the shallow region, the water waves travel more slowly and the wavelength is shorter.
• If the waves approach at an angle, they change direction (bend towards the normal as they slow down).
• If the waves approach head-on (along the normal), they slow down but do not change direction.

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Drawing Refraction Ray Diagrams

When drawing a refraction diagram:

1. Draw the boundary between the two media as a straight line.
2. Draw the normal at right angles to the boundary at the point of incidence.
3. Draw the incident ray from the first medium to the boundary.
4. If the wave enters a denser medium: draw the refracted ray closer to the normal (smaller angle).
5. If the wave enters a less dense medium: draw the refracted ray further from the normal (larger angle).
6. Label the angle of incidence and the angle of refraction.

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Refraction of Light Through a Glass Block

When light passes through a rectangular glass block:

1. At the first surface (air → glass): the light slows down and bends towards the normal.
2. Inside the glass: the light travels in a straight line at the slower speed.
3. At the second surface (glass → air): the light speeds up and bends away from the normal.
4. The emergent ray is parallel to the incident ray but is laterally displaced (shifted sideways).

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Common Mistakes

Mistake	Correction
Measuring angles from the surface instead of the normal	Always measure from the normal
Saying "frequency changes during refraction"	Frequency stays the same
Drawing the refracted ray bending the wrong way	Into denser medium = towards normal; into less dense = away from normal
Confusing reflection and refraction	Reflection = bouncing off; Refraction = passing through with a direction change

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Summary

• Reflection: the wave bounces off a surface; angle of incidence = angle of reflection.
• Refraction: the wave changes direction when it changes speed at a boundary between two media.
• Into a denser medium: wave slows down and bends towards the normal.
• Into a less dense medium: wave speeds up and bends away from the normal.
• Frequency does not change during refraction — only speed and wavelength change.
• If a wave hits the boundary along the normal, it changes speed but does not change direction.
• Always measure angles from the normal, not from the surface.

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Worked Examples — Refraction Calculations and Ray Diagrams