This is an article from Curious Kids, a series for children. The Conversation is asking kids to send in questions they’d like an expert to answer. All questions are welcome – serious, weird or wacky!
How does glow in the dark paint work? – Roman, age 5, Katoomba.
To answer your question, we need to talk about light. This is not an easy thing to do. About 100 years ago, the world’s smartest scientists even argued about what light really is. And they argued for many years.
Light is actually a bunch of tiny things that scientists call “photons”. These little things can travel unbelievably quickly.
How quickly? Well, imagine this: photons can go around the entire world more than seven times in just one second.
When these photons reach our eyes, we see them as light. The more photons there are, the brighter the light.
Photons can come in all the colours of the rainbow. They also hold energy which can turn into heat. This is why it feels warm when the sun shines.
But, not all light is the same. Blue and violet photons both have more energy than red ones, for example.
Now here is a weird thing: there are some types of light that are invisible!
For example, ultraviolet (UV) light, which has even more energy than blue and violet light, is invisible.
Sunlight contains some of this powerful UV light. Because it has so much energy, it can cause a lot of damage, like sunburn, if you get too much of it on your skin.
Another invisible type of light is infrared light. Infrared means “less than red”, so this light has even less energy than red light.
Making paint glow
Many light sources, like the Sun or an old light bulb in your bathroom, glow because they are really hot. Normal glowing, like that of the Sun and a light bulb, requires objects to be really hot for us to see it.
As you already know, you can see glow-in-the-dark paint, but if you touch it, it is just as cold as the bedroom wall. So, the glowing of the paint must be different to the glowing of a light bulb.
The paint has a special kind of glowing called “luminescence” and it can only be created from a few types of material. One such material is what scientists call “luminescent phosphors”, and this is what makes your paint glow. Scientists make luminescent phosphors in the lab by mixing special chemicals together, and then add them to the paint. The paint is then sold to factories and manufacturers who put it on toys, stickers, and even inside colouring pens.
While some things glow all the time, like the sun, glow-in-the-dark paint must be “told to glow”. Just like your parents need to charge their phones every night to make them work, these materials need to be “charged” before they start glowing.
Read more: Curious Kids: Is x-ray vision possible?
In fact, the charging of your glow-in-the-dark paint is done by other types of light. The invisible UV light with lots of energy can charge the special phosphors in your paint and make it glow in your bedroom at night.
There are different types of glow-in-the-dark paint. One type can be charged during the day and can glow for hours in the dark at night. The charging that happens during the day, for example by sunlight, is stored in the paint for some time, just like in the battery of a phone.
This type of paint is called phosphorescent. The other type, called fluorescent paint, only glows while an invisible UV light is turned on to charge it.
You might have heard that some animals can glow. Here’s a video all about that:
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Authors: Thorsten Trupke, Professor of Photovoltaic and Renewable Energy Engineering, UNSW