1The law of reflection
Angles are measured from the normal, a line at right angles to the mirror. The angle of incidence always equals the angle of reflection.
normalangle of incidenceangle of reflection
Lesson 24.2 notes
Reflection
The law of reflection, smooth and rough surfaces, and images in mirrors.
2Smooth and rough
A smooth mirror reflects parallel rays together: specular reflection, which gives a clear image. A rough surface scatters them every way: diffuse reflection, so there is no image.
specular reflectiondiffuse reflection
3Images in a mirror
Rays from the candle reflect into your eye, and seem to come from behind the mirror. The image is as far behind the mirror as the candle is in front.
image
4Walking towards a mirror
As you walk towards a mirror, your image walks towards you. It is always the same size as you, and always as far behind the mirror as you are in front.
5Left and right swap
A mirror swaps left and right. That is why AMBULANCE is written backwards on the front of an ambulance: drivers read it the right way round in their mirrors.
6Periscopes
A periscope has two mirrors, each at 45°. Light reflects off the top mirror down to the bottom one, then into your eye, so you can see over a wall.
periscope
7Still water, rippled water
A still lake is smooth, so it reflects a clear upside-down picture of the trees. Wind makes ripples, the surface becomes rough, and the reflection breaks up.
Explore: Mirror lab
Drag the torch to aim its ray at the mirror, and turn the mirror. Compare the two angles, then try a rough surface or a periscope.
Common mistake: Measure angles from the normal
The angle of incidence is between the ray and the normal, the line at right angles to the mirror. It isn't the angle between the ray and the mirror itself.
Remember
- Angle of incidence = angle of reflection, both measured from the normal.
- Smooth surfaces give specular reflection and a clear image. Rough surfaces scatter light in all directions: diffuse reflection.
- A flat mirror's image is the same size and the right way up, as far behind the mirror as the object is in front, and left–right swapped.
Going further at GCSE