Refraction of Waves

This lesson explains refraction — what happens when a wave passes from one medium to another — including ray diagrams, changes in speed and direction, and a qualitative understanding of Snell's law, as required by the Edexcel GCSE Combined Science specification (1SC0).

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What Is Refraction?

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

Key facts:

• When a wave enters a denser medium (e.g. air → glass), it slows down.
• When a wave enters a less dense medium (e.g. glass → air), it speeds up.
• The frequency of the wave does not change during refraction — only the speed and wavelength change.

Exam Tip: Remember — when a wave changes medium the frequency stays the same. The speed changes and the wavelength changes, but not the frequency.

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Why Does the Direction Change?

Refraction occurs because one side of the wavefront reaches the new medium before the other side. The part that enters first changes speed, while the other part continues at the original speed. This causes the wavefront to change direction.

• If the wave slows down, it bends towards the normal.
• If the wave speeds up, it bends away from the normal.

Transition	Speed change	Direction of bend
Less dense → more dense (e.g. air → glass)	Slows down	Towards the normal
More dense → less dense (e.g. glass → air)	Speeds up	Away from the normal

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Ray Diagrams for Refraction

Light Passing Through a Glass Block

graph LR
    A["Incident ray in air"] -->|"θ₁"| B["Glass surface"]
    B -->|"θ₂ (smaller)"| C["Refracted ray in glass"]
    C -->|"Exits glass"| D["Emergent ray in air"]
    N1["Normal at entry"] -.-> B
    N2["Normal at exit"] -.-> C

When light enters a glass block:

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

Drawing a Refraction Ray Diagram

1. Draw the glass block as a rectangle.
2. Draw the normal (dashed line, at 90° to the surface) at the point where the ray enters.
3. Draw the incident ray hitting the surface.
4. Draw the refracted ray bending towards the normal (for air → glass).
5. At the exit surface, draw another normal and show the ray bending away from it.
6. Label the angle of incidence (θ₁) and angle of refraction (θ₂).

Exam Tip: In your diagram, always draw the normal as a dashed line and use a ruler for straight lines. Mark the angles clearly with arcs. This will help you gain full method marks.

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The Normal Line

The normal is an imaginary line drawn perpendicular (at 90°) to the boundary between the two media at the point where the ray crosses.

All angles in refraction and reflection are measured from the normal, not from the surface.

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Snell's Law (Qualitative)

At GCSE Combined Science level, you need a qualitative understanding of Snell's law:

• The greater the change in speed, the greater the change in direction.
• A material with a higher refractive index causes light to slow down more and bend more.

Material	Approximate Refractive Index
Air	1.00
Water	1.33
Glass (typical)	1.50
Diamond	2.42

The refractive index (n) indicates how much a material slows down light compared to a vacuum.

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Refraction of Water Waves in a Ripple Tank

You can observe refraction of water waves in a ripple tank by placing a sheet of glass or plastic under the water to create a shallow region.

• Water waves travel slower in shallow water and faster in deep water.
• When waves move from deep to shallow water, they slow down and the wavelength decreases (the waves get closer together).
• If the waves approach the boundary at an angle, they change direction — bending towards the normal.

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What Stays the Same, What Changes?

Property	Changes during refraction?
Speed	Yes — changes when entering a new medium
Wavelength	Yes — changes with speed
Frequency	No — stays the same
Direction	Yes — unless the ray hits the boundary at exactly 90° (along the normal)

Special Case: Along the Normal

If a ray hits the boundary along the normal (angle of incidence = 0°), the wave changes speed but does not change direction. The ray passes straight through without bending.

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Worked Example

A ray of light passes from air into glass. The angle of incidence is 40° and the angle of refraction is 25°. Describe what happens to the light.

• The light slows down as it enters the denser glass medium.
• The light bends towards the normal because it is entering a denser medium.
• The angle of refraction (25°) is smaller than the angle of incidence (40°), which confirms the bending towards the normal.
• The frequency of the light does not change.
• The wavelength decreases because the speed decreases (v = fλ; f constant, v decreases, so λ decreases).

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Refraction and Colour

White light is made up of different colours (wavelengths). Each colour is refracted by a slightly different amount when passing through glass or a prism.

• Violet light is refracted the most (shortest wavelength, slows down the most).
• Red light is refracted the least (longest wavelength, slows down the least).
• This is called dispersion and is how a prism splits white light into a spectrum.

Exam Tip: Dispersion questions usually ask you to explain why white light splits into colours. The answer is that different wavelengths are refracted by different amounts.

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Common Mistakes

Mistake	Correction
Measuring angle from the surface, not the normal	Always measure from the normal
Saying frequency changes during refraction	Frequency stays the same
Drawing the ray bending the wrong way	Towards normal when entering denser medium, away when entering less dense
Forgetting to draw normals on diagrams	Always draw dashed normals at each boundary

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Summary