What happens when waves meet a boundary between materials?
Reflection and refraction: how waves are reflected, transmitted or absorbed at a boundary, the law of reflection, and why refraction occurs (separate physics).
A focused answer to AQA GCSE Physics 4.6.1 and 4.6.2, covering how waves are reflected, transmitted or absorbed at a boundary, the law of reflection with specular and diffuse reflection, and why refraction occurs when a wave changes speed.
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What this dot point is asking
AQA wants you to describe what happens when a wave meets a boundary (reflection, transmission or absorption), state and use the law of reflection, distinguish specular from diffuse reflection, and explain why refraction occurs. This is part of topic 4.6.2 of the AQA GCSE Physics (8463) specification.
At a boundary
When a wave is reflected or transmitted, exactly which of these happens depends on the two materials and the type of wave. A mirror reflects almost all the visible light that hits it; a black surface absorbs most of it; a clear glass window transmits most of it. Often a wave is partly reflected and partly transmitted at the same boundary, which is why you can see both your faint reflection and the scene beyond in a shop window. The total energy is always conserved across the three outcomes.
The law of reflection
The normal is central to every reflection and refraction question, because all angles are measured from it, not from the surface itself. A common and costly error is to measure the angle between the ray and the mirror instead. When light reflects off a plane mirror, the image formed is virtual (it appears behind the mirror), upright, the same size as the object, and laterally inverted (left and right appear swapped).
Refraction
The reason refraction bends the ray can be pictured using wavefronts. When a wavefront meets the boundary at an angle, one end of it enters the new material and changes speed before the other end does. This makes the wavefront pivot, so the whole wave changes direction. Because the frequency is fixed by the source, the wavelength must change in proportion to the speed (from ): light entering glass slows down, so its wavelength gets shorter.
Try this
Q1. State the law of reflection. [1 mark]
- Cue. The angle of incidence equals the angle of reflection (measured from the normal).
Q2. Explain what happens to the direction of a light ray as it slows down on entering glass. [2 marks]
- Cue. It refracts, bending towards the normal because it slows down.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksA ray of light travels from air into a glass block. Explain why the ray changes direction as it enters the glass, and state how the direction would change if the ray instead met the surface along the normal.Show worked answer →
The ray changes direction because it slows down as it passes from air into the optically denser glass (1 mark). Because one side of the wavefront enters the glass and slows before the other side, the wave bends; as it slows down it bends towards the normal (1 mark). The frequency of the light stays the same, but the wavelength decreases as the speed decreases (1 mark). If the ray met the surface exactly along the normal (at right angles to the surface), it would not change direction at all, although it would still slow down on entering the glass (1 mark). Markers reward the change in speed as the cause, bending towards the normal when slowing, and the special case of no direction change along the normal.
AQA 20213 marksExplain the difference between specular and diffuse reflection, and explain why a sheet of white paper does not produce a clear mirror image even though it reflects most of the light that hits it.Show worked answer →
Specular reflection occurs at a smooth surface, where parallel incident rays are reflected so that they remain parallel, producing a clear image (1 mark). Diffuse reflection occurs at a rough surface, where the surface is uneven so parallel rays are reflected in many different directions and scattered (1 mark). A sheet of white paper reflects most of the light but its surface is microscopically rough, so the light is scattered in all directions by diffuse reflection rather than reflected as a parallel beam, which is why no clear mirror image forms (1 mark). Markers reward the smooth-versus-rough distinction, the parallel-versus-scattered outcome, and applying it to the rough paper surface.
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Sources & how we know this
- AQA GCSE Physics (8463) specification — AQA (2016)