Frozen Shadows

Use phosphorescence in your favour—strike a pose in front of this special wall and a flash of light will cause your shadow to freeze behind you.

Explanation

The wall is coated with a phosphorescent material. When exposed to light, it absorbs energy and becomes ‘excited’. It then releases the energy (de-excites) relatively slowly and you see this as a ghostly glow.

Your body blocks light from hitting part of the wall, leaving your shadow frozen on it. The rest of the wall is glowing because it has been excited and is now releasing its energy as a glow.

If you’ve seen ‘glow in the dark’ clock faces or stickers that seem to ‘charge up’ in the presence of light, then release it slowly, you’ve seen phosphorescence at work.

Some emergency exit signs are phosphorescent. If there is a power failure, the sign continues to glow.

Extras for Experts

Phosphorescence is temperature-dependent. The excited material can only release its energy when it has been given enough energy (through heat) to do so.

Phosphorescence is a form of luminescence, also known as light without heat. Other forms of luminescence include bioluminescence and chemiluminescence.

Bioluminescence is light that is created by a chemical reaction that occurs within a living organism. The protein that is responsible for the glow is called luciferin.

Many organisms are bioluminescent, including bacteria or algae that make waves ‘glow’ as they splash on a beach. Deep-sea fish use bioluminescence to attract prey, fireflies light up their love life with luciferin and many fungi create their own light, possibly to attract animals to help disperse their spores.

Chemiluminescence is light that is created by a chemical reaction. You can see this at work in glow-sticks that are used at parties and for decorations.

Things to Try

Can you time yourself so that your shadow is airborne?
Your shadow gets sharper as you get closer to the screen. This is because light does not diffuse, or spread out, as much when you are close to the screen.

Questions to Ask

Have you seen any glow in the dark creatures in nature? How do you think this works?

Try an Activity

Flashing envelopes
This is an interesting effect best observed in low light (or better yet, utter darkness!)

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Harmonograph
How can friction and motion combine to make a pattern? Watch the harmonograph draw an intricate pattern that can appear to pulsate in places due to Moiré interference. Best of all, you can take one home with you when it’s finished.

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