John Stiles, of Oamaru Intermediate School, asks :-

What makes the dayglow yellow paint on my cycle helmet stand out at a distance?

John Nicholas, a physicist responsible for New Zealand's optical standards at Industrial Research Ltd, Lower Hutt, responded.

The dayglow paint converts some ultraviolet light, which is invisible to our eyes, into yellow light, which is visible to our eyes, thus making the helmet brighter than its surroundings.

Humans are basically blind as their eyes see such a tiny part of the radiations bombarding us. We see light but dont see radio waves, TV waves, microwaves such as in the microwave oven, heat (infrared) waves such as you feel when placing your hand near a hot iron, and, at higher energy than light waves, ultraviolet waves or X-rays.

The sun emits all these sorts of waves and white light consists of only about one tenth of the sun's emissions. We see the white parts of the helmet because visible light from the sun is reflected off the helmet into our eyes and the average material typically reflects around only one fifth of the visible light incident on it. The trick with dayglow paint is that it contains atoms and molecules which absorb higher energy ultraviolet waves and re-emit lower energy visible waves. This process is called fluorescence.

Furthermore, because our eyes are not very sensitive to blue light (hence blue is not a colour used in traffic lights), we can also use fluorescent materials which convert blue light into lower energy green, yellow or red light.

Hence the total amount of visible light coming from the dayglow paint is about six times brighter than the white areas of the helmet and so the helmet is more easily seen by motorists etc.