AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Waves can be reflected at the boundary between two different materials.
Full specification wording
Reflection of waves (physics only): Waves can be reflected at the boundary between two different materials.
A boundary is any place where the material a wave is travelling through changes: air to glass, air to a brick wall, water to the wall of a tank.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 1 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A boundary is any place where the material a wave is travelling through changes: air to glass, air to a brick wall, water to the wall of a tank. At every such boundary some of the wave bounces back into the first material, and that is reflection. The rule is the law of reflection: the angle of incidence equals the angle of reflection, both measured from the normal, the construction line drawn at 90° to the surface at the point where the ray strikes. A smooth surface such as a plane mirror sends a parallel beam back as a parallel beam, called specular reflection, so a clear image is seen. A rough surface scatters the rays in all directions because the normal points a different way at each point; this is diffuse reflection and no image forms, although the law still holds at every individual point. Sound reflects at boundaries too, which is why an echo returns from a wall.
What earns the marks
- Stating that the angle of incidence is equal to the angle of reflection.
- Measuring both angles from the normal rather than from the surface of the mirror.
- Identifying the boundary as the place where the wave meets a second material.
- Distinguishing specular reflection from a smooth surface from diffuse reflection from a rough surface.
Common mistakes and how to put them right
Measuring the angle between the ray and the mirror surface, so quoting 60° when the angle from the normal is 30°.
Saying a rough surface does not obey the law of reflection, when it obeys it at every point but the normals point different ways.
Assuming the whole wave is reflected, when some is usually transmitted or absorbed at the same boundary.
Calling diffuse reflection refraction because the light appears to spread out.
Exam tips
- Draw the normal first, as a dashed line at 90° to the surface; every angle in the answer is measured from it.
- If a question says different materials, name both, for example air and glass, rather than writing at the surface.
- Use the words specular and diffuse when the question shows a shiny and a matt surface side by side.
Try the lesson
The start of the Reflection of waves (physics only): Waves can be reflected at the boundary between two different materials. lesson, made from checked teaching notes for this specification point.
Have a go first
What happens to a wave when it is reflected at the boundary between two different materials?
Listen
A boundary is any place where the material a wave is travelling through changes: air to glass, air to a brick wall, water to the wall of a tank. At every such boundary some of the wave bounces back into the first material, and that is reflection. The rule is the law of reflection: the angle of incidence equals the angle of reflection, both measured from the normal, the construction line drawn at 90° to the surface at the point where the ray strikes. A smooth surface such as a plane mirror sends a parallel beam back as a parallel beam, called specular reflection, so a clear image is seen. A rough surface scatters the rays in all directions because the normal points a different way at each point; this is diffuse reflection and no image forms, although the law still holds at every individual point. Sound reflects at boundaries too, which is why an echo returns from a wall. A strong answer earns marks for these points. stating that the angle of incidence is equal to the angle of reflection. measuring both angles from the normal rather than from the surface of the mirror. identifying the boundary as the place where the wave meets a second material. distinguishing specular reflection from a smooth surface from diffuse reflection from a rough surface. Exam tip: Draw the normal first, as a dashed line at 90° to the surface; every angle in the answer is measured from it.
Key words
Say what each word means before you turn the card.
boundary between materials
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law of reflection
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normal line
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specular reflection
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See every definition
- boundary between materials
- Any place where the material a wave is travelling through changes, such as air to glass or air to a brick wall.
- law of reflection
- The rule that the angle of incidence equals the angle of reflection, both measured from the normal.
- normal line
- The construction line drawn at 90° to the surface at the point where the ray strikes.
- specular reflection
- Reflection from a smooth surface such as a plane mirror, which sends a parallel beam back as a parallel beam so a clear image is seen.
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