AQA 8463 · Topic 6 · Waves
Reflection: how light bounces and why mirrors work
Move the ray, break the rules on purpose, and test yourself. Each lesson is short, and you can check your understanding as you go.
You will be able to
- Say what can happen to a wave at a boundary
- Draw and label a reflection ray diagram using the normal
- State and use the law of reflection
- Explain specular and diffuse reflection
- Describe how to investigate reflection and measure angles accurately
Other topics
Prototype. Content is written to match the AQA specification but always check against your own notes and the official specification.
Lesson 1
Waves at a boundary
When a wave reaches the edge of one material and the start of another, it does not have just one option. Light, sound and every other wave can do three things.
Waves can be reflected, absorbed or transmitted at the boundary between two different materials. What happens depends on the wavelength of the wave and the properties of the materials.
Try it: what happened?
Pick what happened to the wave. A wrong answer is greyed out so you can try again.
Most objects do not make their own light. We see them because light reflects off them and into our eyes. Things that give out their own light (the Sun, a lamp) are luminous.
Lesson 2
The law of reflection
Reflection follows a rule that is the same for every mirror, every wave and every angle.
angle of incidence = angle of reflection
Explore it
Drag the lamp, or use the slider. Notice the dashed line: the normal.
Incident ray: coming in. Reflected ray: going out. Normal: a line at 90° to the surface where the ray hits. Both angles are measured from the normal.
Measure angles from the normal, not from the mirror. A ray that hits at 20° to the surface has an angle of incidence of 70°.
Try it: predict the angle
Lesson 3
Smooth or rough surfaces
A mirror and a sheet of paper both reflect light. So why can you see your face in one but not the other?
Switch the surface
Specular reflection
From a smooth surface. The normals all point the same way, so parallel rays stay parallel. You see a clear image.
Mirrors, still water, polished metal.
Diffuse reflection
From a rough surface. The surface is uneven on a tiny scale, so the normals point in many directions and the light is scattered. No clear image, but you can see the object from any direction.
Paper, cloth, walls, most everyday objects.
Diffuse reflection does not break the law of reflection. Every single ray still has angle of incidence = angle of reflection. It is just that the normals point in different directions.
Try it: specular or diffuse?
A white surface reflects all colours of light. A black surface absorbs most of it. A red object reflects mostly red light and absorbs most other colours, which is why it looks red.
Lesson 4
Investigating reflection
A typical school practical: does the angle of reflection really equal the angle of incidence? Try a virtual version first, then read how to do it for real.
Virtual practical
Set an angle, then press Record reading. The measurement has a small reading error, just like a real protractor.
Results
| Angle of incidence, i (°) | Angle of reflection, r (°) |
|---|---|
| No readings yet | |
Readings of r include a reading error of up to 1°.
Graph of r against i
The dashed line is r = i.
What do you conclude?
Record at least five readings, then choose the best conclusion.
Doing it for real
- Stand a plane mirror upright on a sheet of paper and draw a line along the back of it.
- Draw the normal at 90° to the mirror, then lines from the normal at measured angles with a protractor.
- Shine a thin ray from a ray box along one line, so it hits the mirror where the normal meets it.
- Mark the reflected ray with two pencil crosses. Remove the mirror, join the crosses, and measure the angle from the normal.
- Repeat for several angles of incidence and record i and r in a table.
To improve accuracy: use a thin ray, a sharp pencil, read the protractor at eye level, and repeat each angle.
Lesson 5