[Physics] Why does laser light not affect glowing materials


I have this childrens rubber ball which glows in the dark after it's exposed to light. I "charge" it with a flash light then play with my dogs at night. I thought to try a very intense green laser, and see how the ball reacted.

The laser light had no effect on the balls ability to glow. So I'm left wondering, why does laser light not allow luminescence (maybe not the right word) materials to glow?

EDIT In Response to Answer.

So I tried a little modification. I tried exciting the ball with three different light sources; a "super bright" Red LED, a very very "super bright" white LED and a blue LED of unknown specs (no package, bottom of my kit). I held the ball to each light source (driven with the same current) for the same approximate amount of time and compared the results. The red LED had no effect. The white had a bit of an effect, enough to see dimly in normal room lighting. The blue led had a significant effect, causing a bright glow. This was interesting as the blue LED was the least bright visually. Yay science!

Best Answer

The ball is probably glowing because it has strontium aluminate in, which produces light by phosphoresence. It's a characteristic of phosphorescence that the light emission is quite long lived. This happens because when you shine light onto a phosphor the light promotes it into an excited state that subsequently decays by interactions with the solid lattice into a long lived metastable state. It's this metastable state that decays slowly and emits light as it does so.

Because of this mechanism the light emitted is always a longer wavelength/lower energy than the light you need to excite the phosphor. You don't say what colour light the ball emits, but if it uses strontium aluminate it will be a slightly bluey green colour. The light needed to excite is has to be bluer than the light it emits, and that's why your green laser won't make the ball glow. It has too long a wavelength.

White light from a torch won't contain that much blue light (unless it has a xenon bulb) but the green laser won't have any blue light at all because obviously lasers are monochromatic.

It's also possible that the ball contains zinc sulphide. This isn't as good a phosphor as strontium aluminate, but it's a lot cheaper. If the ball does contain zinc sulphide then the situation is a little more confused because the colour of the phosphorescence is determined by metal additives such as silver and copper. However, the basic principle still applies, that the light needed to excite the phosphor has to be nearer the blue end of the spectrum than the light emitted.

Anyhow, thanks (and +1 :-) for a facinating question and I wish I could give you another +1 for actually doing the experiment with the laser. If you have some time on your hands see if you can find some coloured gels and try shining your torch onto the ball through the gels. You should find that the red gel won't cause any glow while the blue gel will, and at some point in the spectrum there will be a colour where the glow starts.