The violet/ultraviolet LED's are fun, and have been available for a couple of years. Their wavelength is sufficiently short to view many substances that are activated by long-range UV, such as laundry detergent brighteners,"invisible ink" markings on paper money (THAT's how the vending machine knows a five from a one!), and day-glo and black-light actvated materials. They should be treated with some caution, as UV is not good for the eyes. Since the eye's sensitivity isn't good in this wavelength range, one doesn't have a sense of how bright the light actually is or how much damage is being done. That's a good caution for long-wave red lasers as well.
BTW, a blue LED has sufficient photon energy to activate phosphorescent materials. Red and IR (and to a lesser extent, Y and G) LED's "kill" the phosphorescent glow. Several years ago I invented a box containing several LED's that shows these phenomena, and sent it to the 2000 AAPT Apparatus Competition in Guelph. Unfortunately, I couldn't go - just the box. I tried to find it on the AAPT web site just now, but it's inexplicably not there! Wazzuppwiddat? It's now on the 2001 site - I'm sure I sent the thing to Guelph, but my memory might be faulty. Here's the link: http://www.rose-hulman.edu/~moloney/AppComp/2001Entries/e07fr/eeqhfdemo.htm My newer version of the box is smaller with 6 LED's (IR, R, O, Y, G, B) and building Mark III, with V/UV, is on my list of things to do.
Arbor or Educational Innovations, or both, sell a keychain light called the "X light." It contains three LED's arranged and electronically lit up to run through a color-mixing sequence. I recently saw them sold at Spencer Gifts as well. I have also seen some keychains with three colors of LED's integrated into one LED "package." There are some light-up pens that do color mixing with three LED's, and some keychain wands that flash the LED's. The wand looks sort of pink-white when stationary, but the three colors are separated when the wand is waved about -a nice persistence of vision demo.
On another note, it's electrostatics season. We just got in a bunch of new electrostatics kits, and I have forbidden the students to rub the rods on their clothes. The reason? Clothes are coated with anti-static chemicals from those fabric softener dryer sheets - NOT what you want when you're trying to generate static electricity!
Vickie Frohne
(who frequently shops dollar stores for physics "goodies")
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