Reflection and Refraction

This lesson covers the reflection and refraction of waves, including the laws of reflection and Snell's law qualitatively, as required by the AQA GCSE Physics specification (4.6.1). Understanding how waves behave when they meet a boundary between two different media is fundamental to topics including optics, sound, and seismic waves.

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Reflection of Waves

Reflection occurs when a wave bounces off a surface. When a wave hits a boundary (such as a mirror, wall, or the edge of a ripple tank), it is reflected back.

The Law of Reflection

The law of reflection states:

The angle of incidence equals the angle of reflection.

In symbols: angle i = angle r

graph TD
    subgraph "Law of Reflection"
        N["Normal (perpendicular to surface)"]
        I["Incident ray"] -->|"angle i"| P["Point of incidence"]
        P -->|"angle r"| R["Reflected ray"]
        P --- S["Mirror / reflecting surface"]
    end

Key Definitions

Term	Definition
Incident ray	The incoming ray of light (or wave) hitting the surface
Reflected ray	The ray that bounces off the surface
Normal	An imaginary line drawn perpendicular (at 90 degrees) to the surface at the point where the ray hits
Angle of incidence (i)	The angle between the incident ray and the normal
Angle of reflection (r)	The angle between the reflected ray and the normal

Exam Tip: Angles are always measured between the ray and the normal, NOT between the ray and the surface. This is the most common mistake in ray diagram questions. Always draw the normal first as a dashed line perpendicular to the surface.

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Drawing Ray Diagrams for Reflection

To draw a ray diagram for reflection:

1. Draw the reflecting surface (e.g. a straight line for a plane mirror).
2. Draw the normal — a dashed line perpendicular to the surface at the point where the ray hits.
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.
6. Label the angle of incidence (i) and angle of reflection (r).

Reflection in a Plane Mirror

When you look into a plane (flat) mirror, you see a virtual image:

• The image is the same size as the object.
• The image is the same distance behind the mirror as the object is in front.
• The image is laterally inverted (left and right are swapped).
• The image is virtual — it cannot be projected onto a screen because the light rays do not actually come from behind the mirror.

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Specular and Diffuse Reflection

Type	Surface	Result
Specular reflection	Smooth surface (e.g. mirror, calm water)	Parallel rays are reflected as parallel rays; a clear image is formed
Diffuse reflection	Rough surface (e.g. paper, brick wall)	Parallel rays are reflected in many different directions; no clear image

• Specular reflection occurs on smooth surfaces — the normals at every point are parallel, so all rays obey the law of reflection and remain parallel after reflection.
• Diffuse reflection occurs on rough surfaces — the normals at different points are at different angles, so the rays are scattered in many directions.

Exam Tip: Even in diffuse reflection, the law of reflection is obeyed at each individual point. The rays scatter because the surface is uneven, not because the law of reflection breaks down. If asked to explain diffuse reflection, state that the normals at different points are at different angles.

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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 as it crosses the boundary between two media with different densities.

Why Does Refraction Occur?

• When a wave moves from a less dense medium to a more dense medium (e.g. air to glass), it slows down.
• When a wave moves from a more dense medium to a less dense medium (e.g. glass to air), it speeds up.
• If the wave hits the boundary at an angle (not along the normal), the change in speed causes the wave to change direction.

graph TD
    subgraph "Refraction: Air to Glass"
        A["Incident ray in air"] -->|"Angle i"| B["Boundary"]
        B -->|"Angle r (smaller)"| C["Refracted ray in glass"]
        B --- N["Normal"]
    end

Rules for Refraction

Scenario	Speed	Direction
Less dense to more dense (e.g. air to glass)	Wave slows down	Ray bends towards the normal (angle of refraction < angle of incidence)
More dense to less dense (e.g. glass to air)	Wave speeds up	Ray bends away from the normal (angle of refraction > angle of incidence)
Along the normal (angle of incidence = 0)	Speed changes	No change in direction (ray passes straight through)

Exam Tip: Remember the rule: "slow down, bend towards" (towards the normal). When a wave enters a denser medium, it slows down and bends towards the normal. When it enters a less dense medium, it speeds up and bends away from the normal. If it hits the boundary along the normal, it does NOT bend.

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What Changes and What Stays the Same During Refraction

Property	Does it change?	Details
Speed	Yes	Changes as the wave enters a new medium
Wavelength	Yes	Changes — shorter in denser media
Direction	Yes (unless along the normal)	Bends towards or away from the normal
Frequency	No	Frequency stays the same

This is a crucial point: when a wave is refracted, its frequency does not change. Since v = f x lambda, if the speed decreases and the frequency stays the same, the wavelength must also decrease.

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Refraction of Light Through a Glass Block

graph LR
    subgraph "Refraction through a rectangular glass block"
        A["Air"] -->|"Ray enters glass"| B["Glass block"]
        B -->|"Ray exits glass"| C["Air"]
    end

When a ray of light passes through a rectangular glass block:

1. At the first boundary (air to glass): the ray bends towards the normal (it slows down in glass).
2. Inside the glass: the ray travels in a straight line (no change of medium).
3. At the second boundary (glass to air): the ray bends away from the normal (it speeds up in air).
4. The emergent ray is parallel to the incident ray but is displaced sideways (laterally shifted).

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Total Internal Reflection

When light travels from a denser medium to a less dense medium (e.g. glass to air), it bends away from the normal. As the angle of incidence increases:

1. At small angles: the refracted ray bends away from the normal, and some light is also reflected.
2. At the critical angle: the refracted ray travels along the boundary (angle of refraction = 90 degrees).
3. Beyond the critical angle: total internal reflection occurs — all light is reflected back into the denser medium. No light passes through the boundary.