Refraction of Light

This lesson covers refraction of light as required by the Edexcel GCSE Physics specification (1PH0), Topic 5: Light and the Electromagnetic Spectrum. You need to understand why refraction occurs, how to draw refraction ray diagrams, and — for Higher tier — how to use Snell's law to calculate refractive index.

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What Is Refraction?

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

• When light passes from air into glass (a denser medium), it slows down and bends towards the normal.
• When light passes from glass into air (a less dense medium), it speeds up and bends away from the normal.
• If light hits a boundary at exactly 90° (along the normal), it passes straight through without bending — it still changes speed but does not change direction.

Key Definitions

Term	Definition
Refraction	The change in direction of a wave when it crosses a boundary between two media
Normal	A line drawn at 90° to the boundary at the point where the light enters
Angle of incidence (i)	The angle between the incident ray and the normal
Angle of refraction (r)	The angle between the refracted ray and the normal
Optically denser medium	A medium in which light travels more slowly (e.g. glass, water)

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Why Does Refraction Occur?

Light travels at different speeds in different materials:

Medium	Approximate Speed of Light
Vacuum	3.0 × 10⁸ m/s
Air	~3.0 × 10⁸ m/s (very close to vacuum)
Water	~2.3 × 10⁸ m/s
Glass	~2.0 × 10⁸ m/s
Diamond	~1.2 × 10⁸ m/s

When light enters a denser medium at an angle, one side of the wavefront slows down before the other, causing the wave to change direction. This is refraction.

Exam Tip: When explaining refraction, always state that light changes speed at the boundary. Do not simply say "light bends" — you need to explain why it bends by referring to the speed change.

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Drawing Refraction Ray Diagrams

flowchart TD
    A["Draw the boundary between the two media"] --> B["Draw the normal at 90° to the boundary<br/>(dashed line)"]
    B --> C["Draw the incident ray arriving at the boundary"]
    C --> D{"Is light entering a<br/>denser or less dense medium?"}
    D -- "Denser<br/>(e.g. air → glass)" --> E["Refracted ray bends<br/>TOWARDS the normal<br/>(angle of refraction < angle of incidence)"]
    D -- "Less dense<br/>(e.g. glass → air)" --> F["Refracted ray bends<br/>AWAY from the normal<br/>(angle of refraction > angle of incidence)"]
    E --> G["Add arrows on rays<br/>Label angles i and r"]
    F --> G

    style A fill:#2c3e50,color:#fff
    style G fill:#27ae60,color:#fff

Rules for Drawing

1. Draw a clear boundary line between the two media. Label each medium (e.g. "air" and "glass").
2. Draw the normal (dashed) at 90° to the boundary.
3. Draw the incident ray with an arrow showing direction.
4. At the boundary, the ray changes direction (unless it hits at 90°).
5. Going into a denser medium → bend towards the normal (angle of refraction is smaller).
6. Going into a less dense medium → bend away from the normal (angle of refraction is larger).
7. Label the angle of incidence (i) and angle of refraction (r).

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The Glass Block Experiment

A rectangular glass block is used to investigate refraction:

1. The incident ray enters the glass block at the first boundary — it bends towards the normal.
2. Inside the glass, the ray travels in a straight line.
3. The ray exits the glass block at the second boundary — it bends away from the normal.
4. The emergent ray (the ray leaving the block) is parallel to the original incident ray but displaced (shifted sideways).

Exam Tip: In the exam, you may be asked to explain why the emergent ray is parallel to the incident ray. It is because the refraction at the entry surface is reversed at the exit surface — the bending towards the normal on entering is cancelled by bending away from the normal on leaving.

Angles on the diagram: i = angle of incidence (in air), r = angle of refraction (inside glass). On entering the denser medium, r < i — the ray bends towards the normal. At the exit boundary, the reverse happens and the emergent ray leaves parallel to the incident ray, shifted sideways.

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Refractive Index

The refractive index (n) of a material tells you how much light slows down (and therefore bends) when it enters that material.

$n = \frac{\text{speed of light in vacuum}}{\text{speed of light in the material}}$n=speed of light in the materialspeed of light in vacuum​

Material	Refractive Index (n)
Vacuum / Air	1.00
Water	1.33
Glass	~1.50 (varies with type)
Diamond	2.42

• A higher refractive index means light travels more slowly in that material.
• A higher refractive index also means light bends more when it enters the material.

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Snell's Law (Higher Tier)

Snell's law relates the angles and refractive indices at a boundary:

$n_1 \sin \theta_1 = n_2 \sin \theta_2$n1​sinθ1​=n2​sinθ2​

Where:

• n₁ = refractive index of the first medium
• θ₁ = angle of incidence (in the first medium)
• n₂ = refractive index of the second medium
• θ₂ = angle of refraction (in the second medium)

If light passes from air (n ≈ 1.00) into another material:

$\sin i = n \times \sin r$sini=n×sinr

Or equivalently:

$n = \frac{\sin i}{\sin r}$n=sinrsini​

Worked Example 1

A ray of light passes from air into glass (n = 1.50) at an angle of incidence of 45°. Calculate the angle of refraction.

Step 1: Use Snell's law. Since the light is going from air (n₁ = 1.00) into glass (n₂ = 1.50):

$n_1 \sin \theta_1 = n_2 \sin \theta_2$n1​sinθ1​=n2​sinθ2​

Step 2: Substitute values:

$1.00 \times \sin 45° = 1.50 \times \sin \theta_2$1.00×sin45°=1.50×sinθ2​

$0.707 = 1.50 \times \sin \theta_2$0.707=1.50×sinθ2​

Step 3: Rearrange:

$\sin \theta_2 = \frac{0.707}{1.50} = 0.471$sinθ2​=1.500.707​=0.471

Step 4: Find the angle:

$\theta_2 = \sin^{-1}(0.471) = 28.1°$θ2​=sin−1(0.471)=28.1°

The angle of refraction is 28.1°.

Worked Example 2