Wave Behaviour at Boundaries

This lesson covers the different ways waves can behave when they encounter a boundary — absorption, transmission, reflection, refraction, and diffraction — as required by the Edexcel GCSE Physics specification (1PH0), Topic 4: Waves. You also need to understand the core practical for investigating reflection and refraction of light.

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What Happens When a Wave Meets a Boundary?

When a wave meets a boundary between two different media, one or more of the following can occur:

Behaviour	Description
Absorption	The wave's energy is transferred to the material — the wave is "taken in" and its energy is converted (often to thermal energy).
Transmission	The wave passes through the material and continues on the other side.
Reflection	The wave bounces back off the boundary into the original medium.
Refraction	The wave passes through the boundary into the new medium but changes direction (due to a change in speed).

In reality, at most boundaries, a combination of these occurs. For example, when light hits glass, some is reflected, some is transmitted (and refracted), and some is absorbed.

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What Determines Which Behaviour Occurs?

The behaviour of a wave at a boundary depends on:

1. The type of material: some materials absorb certain wavelengths more than others. For example, glass transmits visible light but absorbs ultraviolet.
2. The frequency (wavelength) of the wave: different frequencies interact differently with materials. For example, X-rays pass through soft tissue but are absorbed by bone.
3. The angle of incidence: the angle at which the wave hits the boundary affects whether reflection or refraction dominates (e.g. total internal reflection at large angles).

Examples

Wave	Material	Main Behaviour
Light	Clear glass	Mostly transmitted (and refracted)
Light	Mirror	Mostly reflected
Light	Black card	Mostly absorbed
Sound	Hard wall	Mostly reflected (echo)
Sound	Soft furnishings (carpet, curtains)	Mostly absorbed
Infrared	Human skin	Mostly absorbed (warms the skin)
X-rays	Bone	Mostly absorbed
X-rays	Soft tissue	Mostly transmitted
Radio waves	Brick walls	Mostly transmitted (pass through walls)

Exam Tip: When asked about wave behaviour at a boundary, consider all four options (absorption, transmission, reflection, refraction) and state which is the dominant behaviour for the given situation. In many real-world scenarios, some of each occurs simultaneously.

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Core Practical: Investigating Reflection and Refraction with Light Rays

This is an important practical that you may be asked to describe or evaluate.

Equipment

• Ray box (or a laser pen — if using a laser, follow safety procedures)
• Plane mirror (for reflection)
• Rectangular glass or Perspex block (for refraction)
• Protractor
• Ruler
• Pencil and paper
• Pins (optional, for tracing rays)

Method — Investigating Reflection

1. Place the plane mirror on a sheet of paper and draw around it.
2. Draw a normal line (perpendicular to the mirror surface) at the point where the light ray will hit.
3. Shine a ray of light from the ray box towards the mirror at a known angle of incidence (measured from the normal).
4. Mark the position of the incident ray and the reflected ray on the paper.
5. Measure the angle of reflection (between the reflected ray and the normal).
6. Repeat for different angles of incidence (e.g. 10°, 20°, 30°, 40°, 50°, 60°, 70°).
7. Record results in a table and verify that the angle of incidence = angle of reflection in each case.

Method — Investigating Refraction

1. Place the rectangular glass block on a sheet of paper and draw around its outline.
2. Shine a ray of light from the ray box at an angle towards one face of the block.
3. Mark the position of the incident ray (before the block), the ray inside the block, and the emergent ray (after the block).
4. Remove the block and join the marks to show the complete path of light.
5. Draw the normal at each boundary.
6. Measure the angle of incidence and angle of refraction at the first boundary.
7. Repeat for different angles of incidence.
8. Record results in a table.

Expected Results

Reflection:

• Angle of incidence = angle of reflection (for every angle tested).
• This confirms the law of reflection.

Refraction:

• At the first boundary (air → glass): the light bends towards the normal (angle of refraction < angle of incidence).
• At the second boundary (glass → air): the light bends away from the normal.
• The emergent ray is parallel to the incident ray but laterally displaced.

Sources of Error

Error	Improvement
Difficulty measuring angles accurately	Use a sharp pencil and a large protractor; measure from the normal
Ray is too wide to pinpoint exact angle	Use a narrow slit on the ray box or a thin laser beam
Parallax error when reading the protractor	Read the protractor from directly above
Glass block slipping during the experiment	Fix it in position with Blu-Tack

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Wavefront Diagrams — Showing Refraction

A wavefront diagram shows waves as a series of parallel lines (each line represents a wave crest). Wavefront diagrams are useful for showing refraction:

• When waves move from deep water to shallow water (or air to glass), the wavefront spacing decreases (shorter wavelength) and the wavefronts bend towards the normal.
• When waves move from shallow to deep water (or glass to air), the wavefront spacing increases (longer wavelength) and the wavefronts bend away from the normal.

The part of the wavefront that enters the new medium first slows down first, causing the whole wavefront to change direction.

flowchart LR
    A["Wavefronts<br/>(wide spacing)<br/>Deep water / Air<br/>Faster speed"] --> B["Boundary"]
    B --> C["Wavefronts<br/>(narrow spacing)<br/>Shallow water / Glass<br/>Slower speed"]

    style A fill:#3498db,color:#fff
    style B fill:#f39c12,color:#fff
    style C fill:#2c3e50,color:#fff

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Diffraction

Diffraction is the spreading out of waves when they pass through a gap or around an obstacle. Diffraction is a property of all waves — light, sound, water waves, and EM waves all diffract.

Key Points About Diffraction

• Diffraction causes waves to spread out into the region behind the obstacle or beyond the gap.
• The amount of diffraction depends on the relationship between the wavelength and the size of the gap:

Gap Size vs Wavelength	Amount of Diffraction
Gap much larger than wavelength	Very little diffraction — the wave mostly passes straight through
Gap similar size to wavelength	Maximum diffraction — the wave spreads out significantly in a semicircular pattern
Gap much smaller than wavelength	The wave is mostly blocked (very little energy passes through)

Why This Matters