While messing around with ideas for my talk at the Cheltenham Science Festival, I hit on the idea of combining a piece of acrylic that only passes infrared light, with a cheap Fresnel magnifying lens. Completely by accident, I managed to get the rather pretty effect to the right. You can see that the paper is burning where there is no visible light, in the infrared portion of the spectrum. Even better, it’s quite a cheap effect to achieve.
- An A4 sized flat magnifying lens (sometimes called a “sheet magnifier”). I’ve seen them on sale for £4 or so.
- A filter, made of material that blocks visible light but passes infrared, such as the acrylic sheet pictured to the right (I got mine from ebay for £10)
- Some board or cardboard to hold the lens & filter.
- A nice sunny day, and something to burn (junk mail?)
Assuming your IR-passing filter is smaller than your magnifying lens, you’ll need to cut your lens down to size. Alternatively, you could do what I did, and mask off the light around the filter. I happened to have an old cork pinboard hanging around, so I cut a hole in that, slightly smaller than the IR filter, and then attached the filter to one side of the board. Note the four drill holes in the filter, visible at the top and bottom. I meant to block those up, but couldn’t be bothered. In this case, fortune favoured the ill-prepared.
Face the whole thing towards the sun. You should be able to concentrate the IR in one spot, and burn a hole using invisible light. Normally, the problem is knowing how to focus the lens on just the right spot. That’s where the holes in the filter come in. They act as sort of rainbow-coloured cross hairs: you can just see them in the picture.
If you adjust the lens so that the cross-hairs are almost together, as in the picture at the top of this post, it indicates that the lens is focussing IR light from the entire area of the filter onto the middle of the cross-hairs. On a sunny day, that’s enough to create a fair bit of heat. If you want impressive rather than pretty, you can now cover up the holes, and watch an entirely invisible incineration.
Have fun, but be careful with intensely hot invisible light. It could be dangerous. Oh, and never look at the sun through the lens, of course, even with the filter in place. You’ll probably permanently damage your vision.