Some surfaces are so smooth that the light reflected from them is not scattered off in all directions.

This can be compared to throwing a ball at a wall. If the wall has a smooth surface then the ball will bounce off the wall in a predictable direction. This will be true of all ball thrown at this wall.

It is observed that: angle a = angle b

However, if the wall has a rough surface, the result is not as predictable:

It can be seen that it is still true to say that: angle a = angle b but because the surface is rough and the wall is at all sorts of angles, it is not possible to predict the angle at which the ball will rebound.

Similarly light that is reflected from a ‘rough’ surface is scattered in all directions.

‘Rough’ is on a very small scale with light.

Light that is reflected from a very smooth or polished surface such as a mirror or still water is reflected in predictable conditions.

again angle a = angle b

The above is another example of a ray diagram where the path of the light is represented by a straight line with an arrow.

The fact that rays of light are reflected from a very smooth surface regularly is the reason why we can see ourselves in a mirror. If you look in a mirror, you appear to be behind the mirror but, of course, you are not so what you see is called a virtual image. Children quickly understand this - when we clean our teeth and look in the bathroom mirror, we appear to be behind the mirror but if we excavate the wall we are not there! Virtual really means where something appears to be.

This is best demonstrated with a ray diagram:


The rays of light are extrapolated (extended) back to the point where these rays appear to be coming from. This is the position of the virtual image.

This image is laterally inverted, i.e. you appear the right way up but left and right are swapped.

Demonstration: To help the understanding of lateral inversion have two people standing facing each other, one is pretending to be the image of the other in a mirror. The one moves a right hand and the other moves the hand that would appear to move in a mirror. This is the second person's left hand.

Ideas for primary activities:


Sight, Safety, Light needs a source, Speed of Light
Transparency, Opaque materials and shadows
Reflection of Light, Intensity
Wavelength, Colour, Filters
The eye
Assessment Material
Reference Material