Spark image

The bicycle reflector

If the reflector was simply a flat mirror then the beam would only reflect into their eyes for one position of the car in fact you would only see the bike if it was directly in front of you (a)!

If the bike was to one side the reflection would not meet their eyes and the bike and rider would effectively disappear (b).

However real bike reflectors are not plane. They are made of a large number of small tetrahedrons set together so that there are dips in between in the shape of a corner of a right angled cube.

This is difficult to draw in 3D so we will look at the 2D version where a beam of light hits the corner of a square.

The two diagrams on the right show that whatever angle the light beam hits the corners it will reflect back along its original path. So what actually happens on the road is shown in (c).
© Keith Gibbs 2009