Summer STEM Challenge: Fire from Water

Heating things up is really important in chemistry. But in the early days of the science, this wasn’t easy. Fire was probably a pile of dirty wood or sooty coal in a grate. So pioneer chemists like Joseph Priestley and Anton Lavoisier used nice clean sunshine... and a lens. Not a little one like you keep in your pocket. No, these ‘burning glasses’ were huge, 30cm or more. Lavoisier even had a monster lens 2.5m in diameter. Here is how to make your own great big burning glass – out of water.

As well as water, you’ll need cling-film and a ring, ideally a bit smaller than the cling-film. The wheel-rim from a child’s bike will do nicely, or even a full-size bike wheel-rim. You’ll also need a mirror as wide as the ring and twice as long.

Cover the ring in cling film, trying to make the sheet flat. You can put a piece of elastic – or a chain of elastic bands – around a wheel-rim over the cling-film edges and then pull it taut. Tape over that to make it even more secure. If you are using a wheel-rim bigger than the clingfilm, do it in two halves. Cover one half, put a thin continuous line of glue-like ‘UHU’ down the free edge, and then cover the other half with an overlap that includes the glue. Well-secured cling-film can be flattened by warming with a hair dryer.  

Now put the ring on the edge of two chairs, so that it is horizontal, cling-film up, and pour water onto the cling film. With luck, as you get a puddle in the middle, the puddle will deepen, and as you add more, the water will form a lens-shaped puddle. Put something underneath and look through the water – you will see a magnified image.

The cling film needs to sag the right amount. If it’s sagging too much, try a little less water, or more hair-drier tightening. If not sagging enough is the problem, try using more water, warm water, or even hot water (but not boiling). Or try another sort of cling film.

Now turn the wind off (you don’t want ripples on your lens), switch the Sun on, and adjust the mirror. You’ll see that the Sun concentrated badly off to the side of the ring. Lenses work much better when light goes through them at 90°, which means the Sun needs to be directly overhead, which doesn’t happen in most places. This is where the mirror comes in. Put the mirror on another chair and adjust it so that sunlight goes vertically downwards through the burning glass. Bingo! You have a clean, free source of searing heat.

Be careful with this hellishly hot device. Don’t put it on a flammable surface like decking. Keep a bucket of water handy, and put something over the lens when you’re not supervising. Check where the best focus is. Also, DON’T use your hand. Use something non-flammable like a tile.

More water = shorter focal length f. Smaller f is good because the power concentration increases as (lens diameter/f )². Now a 30cm bike wheel-rim burning glass will put 30W onto its focus. If that focus is 1cm², then it’s 30W/cm², which is 600x more concentrated than sunlight. That’s why it’s HOT.