Ultraviolet Light Illumination

Caution: be very careful about looking at things under certain frequencies of UV light as retinal damage can occur.

Check out "fluorescence" in on-line encyclopaediae.

Thanks PWSlack. I'll check it out.

CLoud

Well I have read a little on fluorescence. From what I can gather is fluorescence is the re-emittion of (light) energy under excitation, in this case ultraviolet. Most fluorescence materials are minerals and organic substance.

PWSlack writes:

" Caution: be very careful about looking at things under certain frequencies of UV light as retinal damage can occur."

Okay, from Stokes's Law, the re-emitted light is always of higher wavelength than the original (that is why I can see the fluorescence by uv). Is the re-emitted light dangerous? Cause as far as I know visible light is considered harmless.

One other thing, lets say a pure, uncontaminated organic substance is subjected to an uv radiation that is very short (i.e. 254nm), is it possible for the substance to re-emit uv radiation, albeit at a longer wavelength (i.e. 300++nm) ?

Exposure to ultraviolet light is one of the many techniques of sterilising water. In summary, certain frequencies of UV light screw up the biochemistry in cells floating in it, thereby killing them and rendering the water sterile. It does the same to the cells in the retina, if one lets it.

"...the future's so bright, I gotta wear shades..."

That much, I figured, but what about the re-emitted light? Can an organic substance re-emit uv (albeit at a higher a wavelength perhaps; i.e. from 200nm to 300nm) ?

Generally, emitted wavelengths are longer than absorbed wavelengths.

Fluorescent materials convert a broad range of illuminant wavelengths into a narrow band; the intensity of the emitted wavelengths exceeds the intensity of those specific wavelengths upon arrival, which is why fluorescent materials appear "brighter" than their background. To experiment with fluorescence, apply a broad spectrum from a slit-and-prism light source, say, to a fluorescent surface, and see what happens. It's most illuminating...

254nm is really high UV! And, yes, there are some things that would re-emit in the lower UV regions. Also, remember that not only are your eyes in danger, high power, high frequency UV is an ionizing radiation. Its wavelength is short enough to damage your cells' DNA! After you get through having fun, a visit to the dermatologist in a year or so might be in order.

the UV light is absorbed by the shining material, the energy is used to bring electrons on higher electron trajectories. When they fall back, photons are discharged which let the material shine at visible wavelengths. Chris

Hi Guest. Not all materials glow under UV light. I use UV light every day, both in long wave and short wave as an analytical tool in mineralogy and gemmology. When a mineral or gemstone is pure it will not normally glow under a UV light, but if it has impurites such as cromium ions or manganese ions within its crystal latice it will glow. The colour it radiates will determine what the impurity is, except if the impurity is Iron or copper for example, then it will be inert. There are three examples where UV light are in every day use. One is in mercury steet lighting where the colour produced is not the yellow of mercury. If the mercury is heated to a vapour the UV light produced will irradiate the inside coating of the bulb which is usually made up of a variety of element oxides, sulphates and salts. These will then glow, giving off visible light. The second is white shirts, when these are washed they will glow bright white. The reason being that the washing powders have phosphates in them which glow under UV light. The third is certain dyes in synthetic firbres, these dyes are usually made from metallic element oxides or phosphates. Spencer.

When you get your light, one thing you gotta try is check out liquid laundry detergent in black light. In normal light, they're a sort of clear liquid, but under UV they're opaque white!

"You're whites are brighter!!!" because you've just put a bunch of dye on them that fluoresces from the UV in daylight. Check out a laundry room in the dark with the lamp. You'll be surprised how much of the stuff gets all over the place - you just can't see it under normal light.

Thank you vermin your suggestion is certainly interesting and worth trying and I know I'm gonna do it but I am just curious, what is the stuffs that is causing the detergent to fluorescent.

And while we are at it, what is the component that distinguish fluorescent materials and enables them to fluorescent. Or is it a state/condition the materials in (as over time fluorescent road signs will "dull" with exposure to sunlight).

I failed to see the connection between biological life, minerals and gems, organic substances, and detergents.

Woof!!! You want this all tied together! Let's see how I can answer that...

I suppose you could define fluorescence (in general) as the ability of electrons within certain chemicals to first absorb light at particular frequencies, and then re-emit the light at other frequencies. Not all chemicals can do this, however, in nature, there are some chemicals (molecules?) that contain electrons that are at just the right quantum states (energy levels) that enable them to absorb light, rise to a higher quantum state (there are no in-between states), and in this way, store the light energy. This is usually a temporary condition, because the electron eventually drops back to its ground energy level. When this happens, the stored energy in the electron's orbit is released as light. Often, the trip back down to the ground state is not a "non-stop" ride. There may be several quantum states on the way down that the electron passes through. Light can be given off at all of these intermediate downward transitions, and the frequency of the light given off can be calculated by knowing the energy deltas between each of these states.

Fluorescence does not happen only when materials are exposed to UV light. As E-Man showed in his picture, the chromium in his ruby rod fluoresces when exposed to green light. This happens because the electron structure of chromium is such that a certain electron in the chromium atom has a quantum state to jump to, whose difference is equal to the energy of a photon of green light. Once the electron decides to come back down to its ground state, it gives back its stored energy as light. Again, the frequency of the light at which the chromium atom fluoresce, is the delta between the energies of the various quantum states that the electron transitions through on its way to ground state. Notice that in this case (and most others) that it's not "Green in - green out" or "UV in - UV out." Remember that the electron may have several energy states to travel through on the way down. And the smaller the energy delta between these states are, the lower the frequency is of the re-emitted light.

So, considering that there are a lot of complex chemicals out there whose electron shells are "tuned" such that they can absorb energy in the form of light at a particular frequency, and then release the energy as they go back to ground state, there are a lot of things that can fluoresce. Conversely, there are a lot more chemicals out there whose electron shells are not "tuned" to particular frequencies of light. These are the chemicals that don't fluoresce.

So whether you're seeing fluorescence in an organic dye, or a mineral, or a type of cell, the mechanism is the same. It just so happens that those things that fluoresce have electron shells that are "tuned" such that they have the ability to absorb light energy at a particular frequencies, and then release that energy in the form of light at a later time.

As far as the road signs are concerned, I'm guessing that after a period of being exposed to harsh sunlight, the molecules that fluoresce are broken down into other molecules that can't fluoresce.

One more point: I believe that there aren't more chemicals that fluoresce under UV than any other frequency of light. UV seems to make a lot of things fluoresce only because being in a dark room, under light that your eye can't see, when something does fluoresce, it's just so damn dramatic!!!

Does this make any sense to you?

Surprisingly though it does (to some extent ^_^ ), Thanks Vermin.

Your insight on the "tuning" of certain molecules and the energy delta as a function for the re-emitted light is certainly "enlightening". and easier to understand than most text on the subject.

I have even did some reading, particularly on the road sign subject. Aside from our harsh weather condition, the cause of the fading (harsh sunlight) is also the object of study in photochemistry.

Quoted from McGraw Hills Professional,

The study of chemical reactions of molecules in electronically excited states produced by the absorption of infrared (700–1000 nanometers), visible (400–700 nm), ultraviolet (200–400 nm), or vacuum ultraviolet (100–200 nm) light. Bond making and bond breaking as well as electron transfer and ionization are often observed in both organic and inorganic compounds as a consequence of such excitation.

In this case, our harsh sunlight has given the road sign enough energy for the molecules to break their bonds or create new bonds that no longer "tunes" as a fluorescent. Am I right?

Though this gives raise to the question what is the difference in reflected light and fluorescent light <not the lamp :)> ? Is it because that fluorescent materials have many "stops" and normal reflection is just one-way?

Regards,

CLoud

Cloud8521,

That's a really good quote!!! Can't put it more succinctly than that. Especially good is the light ranges specified from IR to UV. Notice that if the wavelength gets to short, there's so much energy in the photons that they blast atoms apart. Nothing's left to fluoresce!!!

Yes, you are right. Those chemicals that were "tuned" to be able to fluoresce are breaking down into other molecules that are "tuned" differently.

I think you're pretty close to answering your own last question. Very good. Also, this is something you don't (at least I haven't) see much physics on. For example, when physics describes a mirror, it's usually described in a macro format: "Angle of incidence equals the angle of reflection" and so on. However, on an atomic or subatomic level something quite sophisticated must be happening. For example, a mirror works because the atoms that absorb light at a particular angle, re-emit it at an equal and opposite angle. This is probably heresy, but it is kind of like perfect "all at once" fluorescence! Just remember that everything you see around you appears differently depending on the light source - ever been in a parking lot at night that uses monochromatic lights? Everyone's car looks gray!!!

Perhaps some of the others might want to wade in at this point to explain the quantum mechanics involved in reflection and transmission of light.

"For example, a mirror works because the atoms that absorb light at a particular angle, re-emit it at an equal and opposite angle."

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Hi Vermin,

You're getting warm, but the atoms don't absorb the light in the sense that electronic transitions occur in the material and then re-emit the energy when they return to ground state. Typically this would result in a different color being emitted because such transitions aren't direct, but occur in steps where the electron occupies several different energy levels on its return to ground state. As it's the final transition that results in an emitted photon, and since this transition would have less energy than that of the original photon, it will be of a longer wavelength. I.e., fluorescence at a 'redder' color.

What happens in reflection is that the electric field of the impinging photon causes the material's electrons to oscillate, re-radiating the light as a reflection in the usual sense. These electrons are not necessarily bound to any particular atom - such is the case in metals where you have conduction band electrons flitting about from one atom to the next. At any rate, the phase angles work out in such a way as to produce the reflection at the same angle to the surface normal as the incident wave, but with the opposite sign. In a very real sense, the material is re-radiating the light in the new direction.

But here's a brain-teaser for you: if metals (which have lots of conduction-band electrons that lend themselves to highly specular reflection) make good mirrors, and if metals having higher electrical conductivity (such as silver and aluminum vs lead) make better mirrors, why then don't superconductors make perfect mirrors?

Cheers!

-e

Thank you, E-man! As I said in my posting to cloud8521, I've never really run across explanations regarding the subatomic cause of reflection and transmission of light. Your answer gives me a lot to think about... Can you recommend a good source for this info?

I'll will chew on your brain teaser after I get off the Company dime.

OK, I'm guessing that superconductors make lousy mirrors because the conduction band electrons are bound so loosely that they respond like electrons in a superconductor - the light just scatters them throughout the material because there's not much to hold them into place?

Your world is made brighter every day by using "fluorescent" lamps. Each lamp contains a very small amount of mercury that when excited by the electric current running through the interior of the lamp gives of UV light. The inside surface of the lamp is coated with a material that fluoresces under the UV light and gives off the visible light you see. By the way if you are interested in the 300 to 200 mn range by a black light, this is part of the spectrum that they emit.

Regards Mike

Here's a question for you... What is that white stuff in fluorescent lamps? What chemical(s) do they use?

Nichia, creators of the first really bright InGaN blue LEDs, started out making phosphors for fluorescent lamps. That yellow glob of stuff you see in white LEDs, for example, is little different from some types of phosphors used in fluorescent lamps. Nichia still makes fluorescent-lamp phosphors. Here's a link to their phosphor page.

Cloud writes: " I have been wondering, why does things under illumination "shines" under UV light? Isn't uv invinsible to the naked eye?"

Yes, UV light cannot be seen by the naked eye and cannot be seen because it is blocked by the eye's cornea. Corneal replacement patients can "see" long-wave ultraviolet light until a new cornea is attached. Whether they can see short-wave ultraviolet, I'm not sure. The eye's aqueous humor (the transparent jelly-like substance which fills your eyeballs) probably blocks it. By the way, when you look at a black light - the long-wave kind of UV lamps found in nightclubs, etc. - you'll notice that as long as you're looking straight at it, everything else appears to be in a slightly glowing fog. This is your aqueous humor fluorescing. Under black light it glows a kind of blue-green. The violet glow of a black light is not ultraviolet, but merely violet. The wavelengths these lamps emit cross over into the visible region, but the ultraviolet light itself cannot be seen.

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Cloud writes: "I have tried with a lamp with a wavelength well under the defined "visible light" i.e. at 254 nanometer, and still can perceive some illuminations."

By "perceive some illuminations" do you mean that you see light from this lamp? If so, the light you see is definitely not the ultraviolet part of the lamp's output. Humans can generally see light with wavelengths from 800 nm or so (deep red) through 400 nm (violet). This is equivalent to being able to hear only one octave on a piano. Not a very big range! As others on this thread have pointed out, the wavelength of UV you have here, 254 nm, is well into the UV range and can be very dangerous to the unprotected eye. UV-suppressing safety glasses are available, btw. I highly recommend them as my eyes were temporarily damaged by exposure to short-wave UV. It can be quite painful, as I can fully assure you!

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UV light causes different materials to fluoresce because the UV photons are energetic enough to excite atoms and molecules when they absorb a UV photon. After a short time these lose their energy, not as one chunck, but usually in little steps or pieces. The first few steps do not produce visible light, but serve to slightly heat the material. The final chunk is the one that results in the atom or molecule emitting visible light (usually, but some materials emit light in the infrared, which is also invisible to the naked eye). But as this chunk is of a smaller energy than the original UV photon, it is a different color.

Some materials will fluoresce when illuminated with visible light. Not all materials do this just when you shine a UV light on them. For instance, this ruby absorbs violet and green light, and emits red light as a result. Here I'm illuminating the ruby only with green light:

E-man, are you using a 532nm diode laser on your ruby?

No, that's just a 525 nm LED. I have a 10 mW 532 nm diode laser on backorder. When it arrives I'll post another pic. What you'll see in that pic is a shaft of bright red light emitted from the center of the ruby, rather than the entire ruby fluorescing throughout its volume. Should be an interesting shot.

-e