Science Experiments

How does sun cream work?

Sat, 15th Jul 2006

Part of the show Sun Tans, Fission and Fusion

What you Need

A low power ultraviolet (UV) light in a black box
Washing powder tablets
Piece of white paper
Day - glow pen
Sun cream

What to do

1 - Take a washing powder tablet and put it under the UV light. What do you see? Do the same with the piece of paper and compare it to the washing powder tablet.

2 - Take a day-glow pen and draw a picture on your hand.

3 - Put your hand underneath the UV light. What do you see?

4 - Smear sun cream over half of the picture on your hand and put your hand back under the UV light. What happens now?

What may happen

When washing powder tablets are put under UV light, you can see that some of the powder glows blue. A white sheet of paper will also glow blue under UV. Why? Because the people who make washing powder and paper cheat!

Despite what the adverts say, it is very difficult to transform grubby clothes into dazzling whites with nothing but elbow grease. To get around this problem, washing powder manufacturers add a special substance (called an optical brightener) that absorbs UV light.

Ultraviolet light, a part of the electromagnetic spectrum, is not visible to the human eye and is slightly more energetic than the blue and purple frequencies of visible light. When UV light is absorbed by the optical brightener, some of it is re-emitted but at a lower energy level. This lower energy level is the same as the blue and purple frequencies of visible light, which is why we can see it.

Converting the invisible UV into blue light makes the clothes look brighter and gives the impression of being whiter than white.

However, the light given off doesn't have to be blue. Depending on the substance, the light re-emitted under UV can range from blue, to green and even pink. You can see this in day-glow pens. These desktop staples also convert UV light into a colour which we can see, and even work under UV light from the sun. This is why they're called day-glow: they will work under the natural light of day but not a light bulb at night!

Why does it happen?

If you draw on your hand in day-glow pen and put it under UV light, it will glow blue, pink or green - depending on the pen. If you then cover half the picture in sun cream, the half with sun cream will stop glowing.

The pen stops glowing because sun cream blocks ultraviolet light. If UV light can't be absorbed by the substance in the pen (the equivalent of an optical brightener in washing powder), then it can't be re-emitted at a lower energy level and thus the pen won't glow.

Sun cream can block ultraviolet light by either reflecting or absorbing UV. This is important for human health because when UV light reaches the skin it can cause skin damage, sun burn and in some cases, skin cancer.

But sun cream won't block all of the UV light. The 'factor' of your sun cream is a measure of how good it will be at stopping the UV - theoretically, factor 15 will reduce the amount of UV reaching your skin by a factor of 15, and so on.

So if you want to stop sun burn or prevent embarrassing moments when someone draws on you in a nightclub, slap on the sun cream!

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