Science Questions

Why do glow-in-the-dark objects glow in the dark? James Hibbert

Chris -  This is really clever but it goes back 100 years actually.  People discovered a paint in 1905 called zinc sulphide.  They found that when you shone light on this or something radioactive, radium, it could behave as what’s called a phosphor.  In chemicals like that, what happens with them is that when you put some energy in the form of light the light kicks some of the electrons up to higher energy levels than they would have normally.  Immediately you would expect them to fall back down again but they don’t.  In some cases these electrons stay at this slightly higher energy state for a variable amount of time and then they fall back to their normal energy level.  When they do so they give out the energy they soaked up.  They give it out in the visible spectrum and in most cases this is a sort of greeny colour.  Green is the colour the eye is most sensitive to in the dark.  That’s the colour combination most often used because you don’t have to have such a good effect for it to be visible.

It’s possible to make even better chemicals these days.  There’s one called strontium aluminate which is very, very good at soaking up energy and then very slowly allowing it to ooze out again as visible light.  Yamaha, I think, made a motorbike where the fairing had this around the outside edges so it would soak up light during the day and would have a sort of glow-in-the-dark motorbike at night.  The idea being that this would make it more visible to other drivers at night. Your enlarged visible presence subtends a much greater angle on someone’s retina, so they’re much more likely to see you and the motorbike!

July 2008