Reflection of waves (physics only) — AQA GCSE Physics
Test yourself on Reflection of waves (physics only) with AQA GCSE practice questions.
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Reflection of waves (physics only) explained
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.
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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.
Waves can be absorbed or transmitted at the boundary between two different materials.
Reflection is only one of three things that can happen when a wave meets a boundary. The wave can be transmitted, carrying on into the second material, usually with a change of speed and direction; it can be absorbed, so its energy is transferred to the internal energy store of the material; or it can be reflected. Every real boundary does all three at once, and the shares depend on the material and on the wavelength. Colourless glass transmits all the wavelengths of white light, so it looks clear. Blue glass transmits only blue light and absorbs the red and green, which is why it works as a filter and why objects behind it look blue. A black surface absorbs most visible wavelengths and warms up; a silvered surface reflects most of them. Energy is conserved, shared between the reflected, transmitted and absorbed parts.
Students should be able to construct ray diagrams to illustrate the reflection of a wave at a surface.
A reflection ray diagram is a ruled construction, not a sketch. Mark the point where the incident ray strikes the mirror, then draw the normal there as a dashed line at 90° to the mirror surface. Measure the angle of incidence between the incident ray and the normal with a protractor, measure the same angle on the other side of the normal, and draw the reflected ray as a straight ruled line from that same point. Arrows must be drawn on the rays to show the direction of travel. To find an image in a plane mirror, continue the reflected rays backwards as dotted construction lines; where they appear to meet behind the mirror is the virtual image, as far behind the mirror as the object is in front. Marks are given for the normal at 90°, ruled lines, equal angles and correct arrows.
Students should be able to describe the effects of reflection, transmission and absorption of waves at material interfaces.
These questions give you a context and ask what the wave does at the interface and what follows from it. Work through the three outcomes in order. Reflection sends part of the wave back, giving an echo for sound or an image for light. Transmission lets part of it through into the second material, often with a change of speed and direction. Absorption transfers energy from the wave to the internal energy store of the material, warming it. Ozone high in the atmosphere is a good example: it absorbs all the UVC and nearly all the UVB, but none of the UVA, so the most strongly ionising ultraviolet never reaches the ground and the risk of skin damage is cut. A curtain absorbs and scatters light so a room darkens; foam lining a studio absorbs sound so echoes are reduced.
Your focus
- State the law of reflection and identify the normal, the angle of incidence and the angle of reflection on a diagram of a plane mirror.
- Predict the direction of a reflected ray when an incident ray strikes a mirror at a given angle, measuring from the normal.
- Compare the beam reflected from a polished metal sheet with the beam reflected from a sheet of paper, using the direction of the normals.
Show all 12 objectives
- Describe what happens to white light incident on colourless glass and on blue glass, naming which wavelengths are transmitted and which are absorbed.
- Explain why a black surface warms faster than a silver one when both are lit by the same lamp.
- Identify, for a named boundary, the three possible fates of the incident wave and state that the energy is shared between them.
- Complete a part-drawn ray diagram by adding the normal and the reflected ray for a ray striking a plane mirror.
- Measure an angle of incidence with a protractor and construct the reflected ray at the same angle on the other side of the normal.
- Locate the virtual image of an object in a plane mirror using dotted construction lines behind the mirror.
- Predict which of reflection, transmission or absorption dominates at a named surface and state the observable consequence, such as an echo or a temperature rise.
- Explain, using data on absorption, why the ozone layer reduces the risk from UVC and UVB but not from UVA.
- Suggest a material for a given job, such as lining a recording studio, and justify it by the wave behaviour at its surface.
Reflection of waves (physics only) exam tips
Marking Points
- one mark for stating that the angle of incidence is equal to the angle of reflection
- one mark for measuring both angles from the normal rather than from the surface of the mirror
- one mark for identifying the boundary as the place where the wave meets a second material
- one mark for distinguishing specular reflection from a smooth surface from diffuse reflection from a rough surface
- one mark for saying that white light is transmitted by colourless glass
- one mark for saying that only blue light is transmitted by blue glass
- one mark for saying that all the other colours, wavelengths or frequencies are absorbed
- one mark for stating that absorbed energy is transferred to the thermal store of the material
- one mark for a normal drawn at 90° at the exact point where the incident ray strikes the mirror
- one mark for the reflected ray drawn as a straight line with a ruler
- one mark for an angle of reflection equal to the angle of incidence, within the tolerance given
- one mark for adding correct direction arrows to the incident and reflected rays
- one mark for saying that the ozone layer absorbs all of the UVC and nearly all of the UVB
- one mark for linking greater ionising power to greater risk of harm
- one mark for stating that UVA is not absorbed by ozone, so the ozone layer does not reduce the risk from UVA
- one mark for naming a consequence for people, such as damage to skin cells, sunburn or skin cancer
Examiner 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.
- 💡If a question gives a coloured filter, answer in terms of which wavelengths are transmitted and which are absorbed.
- 💡Name the material on each side of the boundary; at the boundary on its own rarely scores.
- 💡Check whether the question asks about colour, energy, or both, and use the word absorbed for the energy part.
- 💡Take a sharp pencil, a ruler and a protractor into the exam; reflection diagrams are marked on measured angles.
- 💡Label the angle of incidence i and the angle of reflection r so the examiner can see you measured from the normal.
- 💡Always include arrows on your rays, as marks are often awarded for showing the correct direction of travel.
- 💡These are often levels-marked questions, so write linked sentences that move from what the wave does to what a person notices.
- 💡Quote figures from the graph or table you are given, such as 95% of UVB, to lift a level 1 answer to level 2.
- 💡Use all three words, reflected, transmitted and absorbed, only when all three apply to the context given.
Common Mistakes
- 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
- writing that blue glass turns the light blue, instead of saying it transmits blue and absorbs the rest
- saying absorbed light disappears rather than its energy being transferred to the internal energy store of the absorber
- treating transmission and reflection as alternatives, when part of the wave does each at the same boundary
- claiming a filter adds colour to white light rather than removing wavelengths from it
- drawing the normal along the mirror surface instead of perpendicular to it
- drawing the reflected ray from a different point on the mirror to the one the incident ray hits
- sketching the rays freehand so the angles fall outside the examiner tolerance
- forgetting to add arrows to the rays to indicate the direction of wave travel
- saying the ozone layer blocks all ultraviolet, when UVA passes straight through
- describing what the wave does but never naming the consequence the question asked for
- using reflected for light scattered from a matt surface without saying the surface is rough
- claiming absorbed waves are destroyed, rather than their energy being transferred to the absorber