Earth Hour Science

At 20.30 UK time today millions of people all over the world will be taking part in Earth Hour by switching off their lights.  We think this is a great idea not just because we think raising awareness of our energy use is vitally important (we consider some of the challenges in meeting our energy needs in our A Rough Guide to Engineering show), not just because the dark is beautiful and most of us, unless we live in or near a fabulous Dark Skies Preserve don’t get nearly enough of it, but also because there is some great science to be done in the dark. So why not spend some of Earth Hour doing some dark science, then let us know how you get on on our facebook page or by tweeting us @scimadesimple. Here are three of our favourite experiments in the dark to get you started:

1. Minty Fresh Flash

Polo mints. Photo: Andrew Mason (CC-BY)

The experiment:

You need to be in the darkest place you can find for this experiment.  Take a polo mint and snap it in half.  You should see a flash of light as it breaks.  You can also try pulling some sticky tape quickly off the roll or crushing those sugar crystals.

What’s happening?

As the mint breaks the positive and negative charges in some of the molecules are separated.  As they recombine the surrounding air gets ionised, generating a flash of light.  This is known as Triboluminescence.

So what?

It’s not just sweets, sticky tape and sugar, quartz crystals also display triboluminescence.  This has been used by Uncompahgre Ute Indians in night time ceremonies to produce rattles that glow when shaken.

2. Relighting candle

You may not want to spend the whole of Earth Hour in total darkness so why not light a candle?

The experiment:

Candle flame. Photo: Matthew Bowden (CC-BY)

Light your candle.  Have another match ready, then blow the candle out.  Light the new match and hold the flame in the smoke coming from the candle (but not touching the wick).  Watch as the smoke catches fire and the flame travels back to the wick, relighting the candle.

What’s happening?

The smoke from the candle contains bits of unburned candle wax (if the candle burnt completely all that would be produced would be carbon dioxide and water vapour, neither of which are visible).  For a flame three things are needed: heat, fuel and oxygen.  When you put the match flame in the candle smoke you have all three, so the smoke catches fire and the flame burns through the smoke to the wick.

So what?

The more smoky the candle the less completely it has burned (try it with different types of candle to see which works best).  This is important because we want fuel to burn as completely as possible, whether in a power station or burning wood to cook.  In either case the more incomplete the burning the more fuel we have to burn to get the same effect and the more polluting smoke particles are released into the atmosphere.

3. Eerie Eyes

Now you’ve lit your candle, you can use it for this experiment.

The experiment:

Eye shut. Photo: Ruth Ellison (CC-BY)

Look at a candle (or other dim light warning: do not use a source brighter than a candle, you could damage your eyes) with your eyes open for a few seconds, then close your eyes, an ‘after-image’ of the candle will appear.  If you move your eyes around the image should move around with them, and fade after a few seconds.  

What’s happening?

There are light sensitive cells in your eyes called ‘rods’. When light hits these, they are ‘depolarised’, and have to be repolarised before they can be used again. When the image is removed (you stop looking at the light) there is a short period when they are still depolarised, giving the impression that the image is still there.  So, when you close your eyes you can see an after-image of the light while they repolarise.

So what?

This effect can also influence our colour perception.  Staring at a colour and then looking immediately at another will result in a distortion in the second colour as a result of the after-image of the first.
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