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So you've made one of our UV torches? There are all sorts of fun things you can do with it to explore the world of ultraviolet light.
Light is all around us but we sometimes take it for granted. What we think of as light is really only part of a wide band of ‘stuff’ known as the electromagnetic spectrum. This spectrum comprises of x-rays and gamma rays at one end, and radio waves at the other. Between these is a thin band that makes up what we can see, from violet at one end through to red at the other.
Just past red in the spectrum is infrared. Common TV remote controls use this form of light to communicate with the TV. Although our eyes can’t see it, by looking at the end of the remote control through a mobile phone camera or webcam you can make it visible; these cameras are sensitive to wavelengths (colours) of light that our eyes can’t see. As you get further away from the colour red, you reach ‘far infrared’ which all objects emit. These rays can be detected with special thermal imaging cameras that police can use to see criminals in the dark, and scientists can use to look at the heat escaping from houses and other objects.
At the other end of the spectrum is ultraviolet. The flashlights we are building here are producing ‘near ultraviolet’, or ‘UVA’, which is still just visible to our eyes; this is evident by the purple glow from the flashlight. This sort of ultraviolet light is not dangerous unlike the shorter-wavelength ‘UVB’ and ‘UVC’ light which can cause sunburn and skin cancer. It is however just far enough away from the normal visible light we see to show some interesting effects.
There are secret symbols on all sorts of things - UK and foreign bank notes, passports, drivers licenses, credit cards and even stamps (UK 2nd class stamps have a stripe down the middle). Tonic water glows (only slightly though), and you might find other things yourself! Even the marks on (posted) envelopes should glow brightly.
What you're seeing here is an effect called fluorescence. The inks used in these objects absorb one colour of light (in this case UV) and emit another in the visible spectrum.
Highlighter pens also fluoresce under UV light - a fun thing is to draw a skeleton on the back of your hand with a yellow highlighter - it will be pretty hard to see under normal light but will glow strongly with your UV torch.
If you have anything made of glow in the dark material you can write on it using your UV torch held right against the surface. The writing will fade over time. The UV light 'energises' the glow-in-the-dark substance, and it gives out light of a different colour (usually a yellowish green). This is an effect related to fluorescence, called phosphorescence.
Some flowers have UV markings on them - scientists think that this might be to make them more attractive to insects who help pollinate them. Some minerals also fluoresce under UV light and geologists can use this effect to identify them. We've had a quick look outside and haven't found anything ourselves in the UK, but if you do, please do let us know - we'd love to hear.
Even some animals glow under UV light. Many scorpions do, but if you are in the UK, you may unfortunately (or perhaps fortunately) be lacking in having many local scorpions around!
The flashlights that we are building are formed by making a very simple electric circuit. They use a coin cell (battery) and an LED (light emitting diode). It is a special LED that emits light at around 400 nanometres (nm) which is just between the UV and visible spectrum. Normally such a simple circuit wouldn’t be recommended; LEDs generally need a device called a resistor to be connected in series to limit the current so that they don’t quickly burn out. In this circuit however the resistor isn’t needed because the LED draws very little current; mainly because the voltage of the battery (3V) is only just approaching the voltage needed to make it turn on. This means that the LED glows only dimly, but it also means that the battery will last for a very long time; probably for several years in typical use.
The black foam acts just as a spacer for the two plastic cards used on either side, and to hold the battery. The leads of the LED act as a switch; they are normally separated from the battery by the foam, and are pushed towards it when the flashlight is squeezed.
A couple of other suggestions - try looking at washing powders under the light; they usually contain optical whiteners which will fluoresce as well; this is how they make your clothes look "white and brighter!". You can also buy fairly cheap invisible ink pens that glow under UV light. People sometimes use these to security mark their valuables.
Also .... (only for those with a strong stomach) ... some bodily fluids fluoresce under UV light - urine is one of them! If you want to see how well your loos are cleaned, turn off all the lights on a dark night and use your torch to see if you can find anything glowing! (yuck!) It is easiest to see this effect on darker materials such as wood. Do note however that there may be other substances that fluoresce in a bathroom including splatters from some substances like soap.
We would really love to hear if you discover anything under UV light that we haven't mentioned!
All the best,
Tom and the Tech Camp Crew