Light is a form of electromagnetic energy that we humans depend upon to see the world around us. It's produced by light sources, like the sun or light bulbs.  White light contains every color of the rainbow: violet, blue, green, yellow, orange, red, and so on. Most of the light that we use to see the world around us is reflected light; when we perceive a banana as yellow, it's because the skin of the banana reflects mostly yellow light.  Grass reflects mostly green, while a red rose reflects mostly - red!  The sun and light bulbs are examples of light sources, that is, we can see them because they produce light, not reflect it. The light sources in our world enable us to see the rest of the world through reflected light!

Scientists use a model of light that characterizes it by its wavelength; for example, the wavelength of violet light is about 400 nanometers. Red light has a longer wavelength, about 650 nanometers. When we perceive an object to be blue, it's because it reflects primarily light of the blue wavelength. Light sources can produce light in various wavelengths, and most of the ones we use everyday to light up our world produce all of the visible wavelengths at once - these are called white lights. White light contains all the colors we can see, and so when you look at an object illuminated by white light, the color you see is that portion of the white light which is reflected by the object. Objects we perceive to be white reflect all of the visible wavelengths; objects we perceive to be black reflect none of them.

Visible light is only a small part of the electromagnetic spectrum. For example, longer wavelengths than red are invisible to humans; just beyond red, that portion of the spectrum is called infrared.  Light below the wavelength of violet light is also invisible to us; this portion of the spectrum is called ultraviolet (UV).

Some minerals and other objects possess an interesting property called fluorescence. When you illuminate them with white light, they reflect some of it, just as any other object might;  for example, green Fluorite  reflects mostly green light, and so we call it green. However, when you shine a UV light on a fluorescent object, something really neat happens:  the object becomes a light source, rather than just a reflector! When you shine the UV light on a piece of fluorescent green Fluorite, it will appear - blue!  Wernerite is another fluorescent mineral; in white light it is a gray-green, but under UV light it looks - yellow!  Fluorescence is not just a curiosity, but very important: for example, fluorescent lamps and bulbs produce white light because a coating on the inside of the glass tubes fluoresces white - the light produced inside the bulb is actually UV, but that stays inside the bulb - the visible white light is the only light which escapes where we can use it.

Just a bit more technology, then we're done! When you shine a UV light on a fluorescent object, some of the energy is absorbed by the material, and then re-transmitted as visible light - the electrons within some of the atoms which make up the mineral are pushed into a higher energy 'orbit', and as they fall back they give off the visible light which we call fluorescence. The wavelength of the light produced is always greater (lower frequency = less energy!) than that which produced the fluorescence, and that's why we can see it (visible light wavelengths are a bit longer than UV wavelengths).

So that's it: Now (if they ask) you can explain to your friends why the rocks glow under UV light! Of course, if you like, you can forget all that, and just enjoy the show: