Why Do UV Lights Make White Things Glow?

Question

I tought UV light was supposed to be invisible, if so then why do white objects apear brighter ?

Answer 1

mgmirkin has it mostly right, with one exception. Electrons release photons when they fall back towards their lowest energy states. Absorption of UV photon causes the electron to jump up two or more energy levels (it can only have certain energies in an atom). If it falls back the same amount, it releases a UV photon to match the one it absorbed. By if it takes an intermediate drop on the way back, (that's why I specified two or more levels) it releases a lower-energy photon. When these lower-energy photons are in the visible light range, we see fluorescence.

Answer 2

Have you ever seen a glow in the dark toy? The thing that makes them glow is called a phosphor. You'll notice that the light that comes from the toys aren't white/yellow like the light from the light bulb that it absorbed, but usually green or blue. What is going on is the same thing, except for the objects are converting invisible UV into visible light (violet, blue, green, etc..) Some chemicals act as a phosphor for UV light. It absorbs the UV light and then re-emits it as a weaker form of light, such as violet, blue, green, yellow, orange, red, etc.. All those combined make white. A few explanations that I have come across talk about laundry detergent containing phosphors. The ones that are advertised as making whites even whiter, contain them. Since the Sun contains UV, your white shirt will appear whiter because it is converting some of the UV into extra white light. A lot of white clothes or maybe all, may be pre-washed in such detergent. As for ALL things glowing under UV lights that are white. I know that not all things do. One strange thing I have seen, is that in a stack of white paper, some of the sheets didn't glow.

Answer 3

Sheesh guys (other answerers), take some physics...

Current theories, well, semi-current theories (Niels Bohr) suggest a stratification or layering of the atom into a nucleus ringed by electron orbits. Electrons are negatively charged particles, orbiting the nucleus of protons and neutrons.

electromagnetic waves are caused by the oscillation of charged particles in a specific manner and at a certain frequency (how often it waves).

UV is a slightly higher frequency of electro-magnetic radiation than regular visible spectrum light. Now, when the light is absorbed by the atom the energy is taken up by the electrons mostly, I think (it's been a while), this causes them to get excited or more energetic, jumping up an orbit or two in the shells of the atom. It's also posited that these "jumps" are what causes photons or electromagnetic light waves to be emitted.

So, when the electrons jump up an orbit or two, they emit a certain amount of energy generally less than the UV that was put into the system.

Consider just as a really oversimplified example: Uv light is 10,000 units of energy, blue light is maybe 5000 units and red light is maybe 2500 units.

So, let's say one UV wave hits an atom and 3 electrons jump from one orbit to another. One of them maybe jumps up 2 levels expending or releasing 5000 units, thus a blue wave, and two electrons jump 1 level emitting waves of 2500 units each. Thus you get 1 blue light wave and two red light waves emitted due to the UV wave hitting the atom. Now different elements might react in slightly different ways emitting slightly different colors out of the spectrum.

So, consider a UV light emitting millions and millions of units of light energy, hitting millions and millions of atoms in a "white" piece of paper. All the electrons will absorb and emit various frequencies or amounts of light. "white" light is actually a mishmash of ALL the visible spectrum of light. IE, we see a little bit of red, a little green, a little blue, a little yellow, etc.

Hopefully this makes sense, the different elements in the paper or cloth will emit different frequencies or colors of light, but we see them all at the same time and call them "white." I think it's just basically 'cause there's too much info for us to distinguish each individual frequency so we just kind of average it all and get "white."

But you'll notice that some clothes glow yellow or glow blue under UV lights. That's because the specific elements are better emitters of those specific colors when they get hit by energetic UV light. IE their electrons bounce in a specific way that mostly produce that specific color you see.

Make sense? Hope so!

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Postscript:
Thanks injanier. Like I said it's been a while, and this is way oversimplified.

Right. They give off photons as they shed a certain amount of energy, thus creating a photon with approximately that amount of energy, and the electron falls back to its regular state from its higher orbital position... Ohh, right and if it falls the same amount it rose it'll emit the same frequency of light, but if it takes intermediate stops on the way it emits photons at those frequencies. So maybe it will release 1) a UV photon 2) a blue photon and 2 red photons 3) 4 red photons (in the oversimplified example above). Depending on how/when the electrons fall back to their original positions. All at once, or in small steps.

Answer 4

Think of the light spectrum..ROYGBIV: Red, Orange, Yellow, Green, Blue, Indigo and Violet.

Now, all colors combined make BLACK. Whereas the absence of all colors is white.

The UV light reflects all colors of the spectrum, thus giving the specimen a white appearance.

Answer 5

Certain things, like white paper, glow because they actually have florescent substances in them that are revealed more strongly by the ultraviolet light. See the attached link for more info.