Introduction: Ultraviolet Torch
There are all sorts of learning objectives you can teach with this one from early primary through to late secondary:
- Light & shadow, color and vision
- Electric circuits - basic series circuits, circuit symbols and diagrams, switches, LEDs and batteries
- Microbes and diseases - transmission of diseases, hand washing and infection, bacteria
- Atomic theory, the atomic model and electron shells, fluorescence and phosphorescence
Step 1: What You'll Need
- A couple of blank PVC ID cards
- A 3mm ultraviolet LED
- Some 5mm (or 1/4") thick foam of some sort that squishes and returns to shape again
- 1 x CR2032 lithium battery
I've put together a few links below of suggested suppliers and parts - with postage costs and minimum quantities you're probably looking at $2-$3 each if you order them in 100+.
If you do the build yourself, this is what you'll need:
- ID Cards: http://www.smartcardsupply.com/Content/Cards/cards.htm - the cheapest blank ones here work out at about 15c each in packs of 100 and you'll need two for each card. These are pretty standard world-wide.
- CR2032 Batteries: http://search.digikey.com/us/en/products/CR2032/P189-ND/31939 - these ones are 28c each
- 3mm UV LEDs: http://search.digikey.com/us/en/products/UV5TZ-395-30/492-1321-ND/2407229 - about $1 each if you get 25 or more. Something from 390nm to 405nm works OK but the lower the wavelength the better.
- Foam: http://mdbuyinggroup.com/products/podiatrist-wholesale/plastazote%C2%AE-14-medium - about $11 which would be enough to make 100. I used 5mm, but 1/4" works as well. There may well be cheaper foams - type is not crucial as long as it is about this thickness - just needs to be able to be squished and return to its original shape.
Step 2: The Foam Insert
Cut out a foam rectangle a few mm shorter on each side than the credit card, and cut a 20mm diameter hole into the middle of one end as shown - exactly where isn't crucial, but it should be close to one end. A scalpel will help with this, although with younger kids scissors are best. I cheated and did my prototypes like this, but ran off the rest on my laser cutter. You can also cut out a notch as shown about the same width as the LED, but this isn't crucial. To make it easier if cutting out by hand, it actually works just as well if you make the 20mm cutout a square (with sides 20mm) rather than a circle. Have also attached a PDF of the pattern I used if that helps anyone.
Now you'll need to stick the foam onto the card - I used two strips of special high-strength double-sided tape, but for most people a glue gun will be a better choice. It works best just putting adhesive along the two long edges I've found.
Step 3: Put in the Battery
Now put in the coin cell into the cutout in the foam. Don't worry about which way up you put it at the moment.
Step 4: Attach the LED
You will need to pull apart the legs of the LED a little, but trying to keep them roughly parallel. They need to be about 5mm (1/4") apart, but this is not super critical.
Now push one leg underneath the cell (will go between the card and foam), and the other leg should go above the cell.
If you have done it correct, when you press down on the leg which is on top of the cell, the LED should glow purple.
If it doesn't glow at all, pull it out, rotate it 180 degrees (so the opposite leg is on the bottom), and put it back in.
If it glows all the time (without pushing), you will need to pull it out, spread the legs further apart, and put it back in again.
Step 5: Complete the Other Side
Now stick the other card down in a similar way to the first one.
Step 6: Investigating Fluorescence
Now its time to use the cards - if they have been constructed properly, when they are squeezed at the end of the card near the LED they will turn on.
Children love looking at secret marks with these. Look at foreign bank notes, passports, drivers licenses - even a 2nd class (UK) stamp has a hidden mark on it. Tonic water fluoresces (only slightly), credit cards are great as well and usually have a couple of secret marks on.
Also try looking at the printed marks on (posted) envelopes - they are often printed in a fluorescent ink that helps the sorting machines work out where they are going.
There are countless other fun things to look at - some minerals will fluoresce, some vitamin tablets and even petroleum jelly (vaseline) glows, albeit not very brightly.
With younger children I introduce this by looking at the spectrum and the colors of the rainbow. We then explore the two 'colors' that are just beyond our vision past the red end and violet end respectively; ultraviolet and infrared.
With older (high school) students looking at the atomic model, this is a great opportunity to look at electrons - what we're doing here is actually exciting electrons to a higher quantum state; when they relax to their ground state they then release a photon of light, typically at a longer wavelength, which conveniently for us is now in the visible range so we can see it brightly. The LEDs we are using here are actually on the end of the visible range, hence why we can see the purple light coming off, but we can still consider the light as being ultraviolet. You can read more about fluorescence here.
The type of UV light we are using here is not dangerous at all - it is only the shorter wavelengths that cause sunburn and skin cancer. I consider this project safe enough for any age student as long as they are old enough for the small parts not to present a choking hazard.
Step 7: The Mock X-ray Machine and More About Fluorescence
This is purely a trick, albeit one that children love. I normally draw a picture of a skeleton on the back of my hand using a yellow highlighter. This is only just visible in normal light, but glows strongly under UV, particularly in a darkened room. If you are really up for some mess, give the children some highlighter pens to play with!
You can do some further explorations into UV by looking at normal copier paper and white clothing. Copier paper and washing powders both have fluorescent substances added to make them glow slightly and appear 'whiter than white' when there is UV present (as there is outdoors). This knowledge can be used to extend investigations into the comparison of washing powders to see if they are really cleaning the best, or are getting a helping hand by using fluorescence.
Step 8: Microbes & Disease
One activity which is really fun is to see how well you wash your hands. You can get a cream that you rub onto your hands before washing them (http://www.glitterbug.com). Anything left afterwards (and there will probably be a surprising amount) will glow strongly.
I usually try to do this with a stronger professional UV torch although it will still work well with these ones in a darkened area.
Your students will be amazed at how badly they wash their hands - areas such as fingernails and backs of hands are often glowing pretty strongly!
You can also make a great demonstration of disease transmission by using the glitterbug products and getting children to shake hands - again, stronger UV lights help particularly after two or three handshakes.
Step 9: Forensics
When it is really dark, go hunting in your bathroom to see how well it is being cleaned ... certain bodily fluids (like urine) fluoresce under UV light. To do this effectively with such a weak source of UV it must be completely dark, and you will see the effect the most on dark areas that are not themselves fluorescent (such as a wooden toilet seat).
Scientists also use UV light to look for blood stains and finger prints using other chemicals, although for some fun science investigations you can easily mock up both of these using the glitterbug powder or lotion mentioned in the last step, creating hidden finger prints and blood spatter (perhaps for older students!).
Step 10: Energizing Glow in the Dark Material
If you've got anything that glows in the dark, you'll be able to energise it with the torch (make it glow). If you hold it really close (with the LED touching it, you will probably be able to 'draw' with light!
This is just plain fun for younger children, but can be used to great effect talking about light and shadows. By placing objects over the sheet and shining the light on them you can create 'semi-permanent shadows.' With older children you can look at Phosphorescence .
Step 11: Microbiology
Some types of bacteria glow under UV light, although you will probably need to culture them properly in a petri dish as small quantities on surfaces, etc will not fluoresce.
This is more suitable for high school and university investigations.
Step 12: The Natural World
Theoretically you could also find many types of scorpions with this light, as they fluoresce as well! Many types of flowers have hidden marks in the UV region, presumably to attract insects that are able to see these wavelengths.
Step 13: Other Experiments
Get yourself a UV pen to security mark your valuable at home or just to write messages in invisible ink. If you do want to use this for secret messages, it works great on the back of one of the blank PVC cards. Standard paper has whiteners added that make it hard to see the message - you might have more luck with brown paper.
It's great fun making this into a 'Spying & Invisible Ink" session. User Johenix suggested a great variation (pictured above) which has a white LED on one end and a UV on the other. One side of the card has a table for morse code printed on. This would make a great 'spy card' for kids to make.
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