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AQA GCSE Physics · Topic 4.6 Waves · Physics only
Reflection of Waves: Ray Diagrams, Mirror Images, Specular and Diffuse
Physics only Combined Science students don't need this page.
In this lesson you'll learn to: state the law of reflection (i = r), draw a reflection ray diagram with a normal, draw a two-ray diagram that finds the image in a plane mirror, explain why that image is virtual, and tell specular from diffuse reflection.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
In this lesson you'll learn to: state the law of reflection (i = r), draw a reflection ray diagram with a normal, draw a two-ray diagram that finds the image in a plane mirror, explain why that image is virtual, and tell specular from diffuse reflection.
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Start here — have a play
Don't read anything yet. On tab 1, slide the angle: does r ever differ from i? Tab 2 builds the mirror-image ray diagram one step at a time, the way you should draw it in the exam. On tab 3, roughen the surface and watch the reflected rays scatter.
Angle of incidence
40°
The law of reflection
Bulb distance from the mirror
Surface
or slide it
Physics with Kate · physicswithkate.com
▲ Three tabs: the law of reflection, the image in a plane mirror, specular and diffuse reflection
Seen how the image is found? Now learn to draw it.
The notes below give the law of reflection, a checklist for every ray diagram, the image-first method for a mirror, and the specular and diffuse definitions.
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Revision notes
What happens when a wave meets a boundary
At the boundary between two materials, a wave can be reflected (it bounces back), absorbed (its energy is transferred to the material) or transmitted (it passes through, and may refract). Which happens depends on the wavelength and the materials, and often it's a mix of all three.
The law of reflection
⭐ Angle of incidence = angle of reflection
i = r. Both angles are measured from the normal: a line at 90° to the surface where the ray hits it. Draw the normal as a dotted line.
A ray at 40° to the normal reflects at 40°.
The trap: the angle between the ray and the mirror is not the angle of
incidence. If a ray makes 50° with the mirror, i = 90° − 50° = 40°.
Drawing a reflection ray diagram
- Use a ruler and a sharp pencil.
- Draw the normal as a dotted line at 90° to the mirror where the ray hits it.
- Put one arrow on the incident ray (towards the mirror) and one arrow on the reflected ray (away from it).
- Make sure each ray touches the mirror, measure with a protractor from the normal, and label i and r.
The image in a plane mirror
⭐ Where is the image?
The image is the same perpendicular distance behind the mirror as the object is in front. It is virtual: the rays only appear to come from it. No light really travels behind the mirror, so the image can't be shown on a screen.
The image-first method gives you i = r without measuring a single angle:
- Measure the perpendicular distance from the object to the mirror. Put a dot the same distance behind the mirror and label it I.
- Draw a ray from I to the mirror. Behind the mirror it is virtual, so make it dotted. Once it passes the mirror, carry on with a solid line and one arrow pointing away from the mirror.
- Draw a second ray from I at a different angle the same way: dotted, then solid with an arrow.
- From the object, draw a straight line to where each ray meets the mirror, with one arrow on each.
- Add a dotted normal at each point. The angle of incidence equals the angle of reflection every time.
Two rays from the bulb into the eye. The image is d behind the mirror.
Specular and diffuse reflection
| Specular reflection | Diffuse reflection | |
|---|---|---|
| Surface | smooth, e.g. a mirror or still water | rough, e.g. paper or a wall |
| Normals | all point the same way | point in different directions |
| Reflected rays | reflected in one direction | scattered in different directions |
| Image? | yes, a clear image | no image |
Specular: one direction.
Diffuse: every ray obeys i = r with its own normal.
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How to write it in the exam
Every mirror-image diagram, the same order
Image first: dot I the same perpendicular distance behind →
two rays from I, dotted behind the mirror, solid with an arrow in front →
object to the mirror with an arrow, then dotted normals.
find the imagethe rays from Ithe real rays
Workbook page 8 · measure i, then draw the reflected ray
A ray hits a mirror at 40° to the normal. Draw the reflected ray and give both angles.
- Angle of incidence i = 40°; angle of reflection r = 40°, measured from the normal on the other side.
- One arrow on each ray, the normal dotted, i and r labelled.
Exam-style question · page 8 · 6 marks
Figure 1: three surfaces, A, B and C.
1. Which one of the surfaces in Figure 1 shows diffuse reflection? (1)
- A: the rough surface scatters the rays. (B is specular reflection; C is refraction.)
Pepper's ghost: the glass acts as a mirror.
2. A light bulb and a thin sheet of glass are inside a box. The student can't see the bulb but sees an image of it ('Pepper's ghost'). (a) Complete the ray diagram and mark the image. (4)
- Image marked behind the glass, the same perpendicular distance from the glass as the bulb.
- A ray from the bulb to the glass, reflected into the eye, with i = r.
- Arrows on the rays, from the bulb towards the eye.
- Dashed lines from the glass back to the image.
(b) Why is the image virtual? (1)
- The rays of light only appear to come from the image: they don't actually pass through it, so it can't be shown on a screen.
📘 Now do it in your workbook
Page 8: measure i and draw the reflected ray, draw two rays from a bulb into an eye and mark the image, draw the reflected rays off a smooth and a rough surface, and a 6-mark exam question on Pepper's ghost.
AQA GCSE Physics Workbook · WAVES · page 8The full workbook covers the whole of Topic 4.6 Waves, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
Don't forget to hit “Mark complete” at the bottom of the lesson.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
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Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.
Now put it into practice
Page 8 of the workbook has the ray diagrams to draw, specular and diffuse reflection and a 6-mark exam question, with answers at the back.
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