What happens to a wave at the boundary between two materials?
Reflection, refraction, transmission and absorption: what happens to waves at a boundary, why refraction changes the direction and speed of a wave, and the wavefront explanation.
A focused answer to Edexcel GCSE Physics 4.9 and 4.10, covering what happens to waves at a boundary between materials (reflection, refraction, transmission and absorption), why refraction changes a wave's direction and speed, and the wavefront explanation of refraction.
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What this dot point is asking
Edexcel statements 4.9 and 4.10 want you to describe the effects of reflection, refraction, transmission and absorption of waves at the boundary between two materials, and to explain how waves are refracted at a boundary in terms of the change of direction and the change of speed.
What happens at a boundary
When light hits a glass window, some is reflected (you see a faint reflection), most is transmitted (you see through it), and a little is absorbed. The proportions depend on the materials and the wavelength of the wave. Absorption transfers the wave's energy to the material, which is how, for example, microwaves heat food and dark surfaces warm in sunlight.
Refraction: the change of speed
The key chain of reasoning Edexcel rewards is: change of medium causes a change of speed, which (at an angle) causes a change of direction. The wavelength also changes (it shortens in the slower medium) while the frequency stays the same.
The wavefront explanation
The wavefront model gives the deeper "why". Because one side of each wavefront slows (or speeds up) before the other, the wavefronts rotate, steering the ray to a new direction. This is the explanation examiners ask for at Higher tier, and it underpins how lenses and prisms work.
How Edexcel examines this
This dot point is examined on both tiers. The boundary-outcomes idea is often a short question asking you to name and describe what can happen to a wave at a boundary, where the mark scheme rewards reflection, refraction/transmission and absorption with brief correct descriptions. Refraction is examined more deeply, frequently with a ray diagram: you may be asked to state which way a ray bends entering or leaving glass or water, where the rule (towards the normal into a slower medium, away from the normal into a faster one) must be applied correctly. At Higher tier, the explanation question asks you to account for refraction in terms of the change of speed and to refer to wavefronts; the full-mark answer states that the medium change alters the speed, that one side of the wavefront slows before the other so the wavefronts pivot, and that this changes the direction. Examiners penalise the loose claim that "the light bends because the glass is denser" without the speed-and-direction reasoning, so always make the speed change the cause.
Try this
Q1. Name what happens to a wave when its energy is transferred to a material at a boundary. [1 mark]
- Cue. Absorption (the wave is absorbed).
Q2. State which way light bends when it enters glass from air at an angle. [1 mark]
- Cue. Towards the normal (because it slows down).
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20213 marksDescribe what can happen to a wave when it reaches a boundary between two different materials. Name three possible outcomes.Show worked answer →
At a boundary a wave may be reflected (it bounces back off the boundary) (1 mark), refracted and transmitted (it passes into the new material, changing direction if it changes speed) (1 mark), or absorbed (its energy is transferred to the material, often warming it) (1 mark). Markers reward naming three of reflection, refraction/transmission and absorption, each with a brief correct description. Listing only "it bounces or goes through" without naming the processes earns fewer marks.
Edexcel 20224 marksExplain, in terms of the speed of the wave, why a ray of light changes direction (refracts) when it passes from air into glass at an angle. Refer to the wavefronts in your answer.Show worked answer →
When light passes from air into glass it slows down because glass is optically denser (1 mark). Because the ray meets the boundary at an angle, one side of each wavefront enters the glass and slows down before the other side does (1 mark). This makes the wavefronts pivot, changing the direction of travel so the ray bends towards the normal (1 mark). On leaving the glass into air it speeds up again and bends away from the normal (1 mark). Markers reward linking the change of speed to the change of direction, the wavefront pivoting because one side slows first, and bending towards the normal on entering the denser medium.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Physics (1PH0) specification — Pearson (2016)