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Author Topic: Black light, what's all that about?  (Read 3573 times)

paul.fr

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Black light, what's all that about?
« on: 24/10/2007 13:55:10 »
the other day i soaked some spinach in 95% ethanol, the next morning i drained the solution in to a beaker and looked at the leaves under a black light. Guess what, the leaves glowed red!

why did that happen?


ooh, should i break my black bulb, could i just paint a normal bulb black?


 

lyner

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Black light, what's all that about?
« Reply #1 on: 24/10/2007 14:07:44 »
'black light' is what we call UV when it's produced with an electrical bulb. The bulb is just covered in a layer of filter which cuts out visible  light. The bulb looks black but UV gets out.
The red glow was fluorescence. This occurs when high energy photons (e.g. UV) are absorbed by the atoms in a substance and, instead of them re-emitting photons of the same frequency, they lose the energy in two stages. One of the transitions corresponds to a frequency that is visible.
Didya know that washing powders ( or at least they used to) contain fluorescent chemicals that produce blue light in UV. This makes the whites look whiter than white.
 

Offline kdlynn

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Black light, what's all that about?
« Reply #2 on: 24/10/2007 15:55:41 »
also makes your clothes look funny under a blacklight if not rinsed properly...
 

paul.fr

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Black light, what's all that about?
« Reply #3 on: 24/10/2007 16:05:56 »
so what's the difference between black light and a uv bulb use in ,say a sunbed?
 

Offline elegantlywasted

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Black light, what's all that about?
« Reply #4 on: 24/10/2007 16:07:30 »
Oh yah detergents still add phosphors to your clothes. In my lighting tech class we did an experiment with black light and various peoples clothing. If you wear black clothing definatly invest in a dark clothing wash, the difference under a black light is crazy!
 

Offline lightarrow

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Black light, what's all that about?
« Reply #5 on: 24/10/2007 17:28:24 »
so what's the difference between black light and a uv bulb use in ,say a sunbed?
The difference is that UV bulbs in a sunbed are transparent to visible light too, so you see also the violet and blue which are emitted together with ultraviolet. So, to make a "black light" bulb, you only have to cover a sunbed bulb with a paint which transmit UV but absorbs violet and blue (at least).
P.S. If I knew which paint could it be, I would have told you!  :)
 

Offline techmind

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Black light, what's all that about?
« Reply #6 on: 24/10/2007 18:05:46 »
... black light ...uld i break my black bulb, could i just paint a normal bulb black?

As others have said, "black light" is an informal term for a UV light source which does not also emit (much) visible radiation. Possibly I'm overstretching the mark, but "blacklights" usually produce the longest-wavelength UV (i.e. only just beyond visible violet) and so are fairly harmless.

Most common UV lamps are based upon a mercury discharge tube, which has several strong emission lines in the ultraviolet: 365nm (near-UV, UV-A, reasonably "safe", used for theatrical/entertainment fluorescent effects), and 254nm which is shortwave "UV-C" which is blocked by glass so needs a more costly quartz envelope and is used for "germicidal" lamps and water purification systems - it'd probably also damage the skin and eye.

Mercury lamps also give off a pale bluey-white visible light: "actinic" (like sunlight) lamps for sunbeds and also for fishtanks often don't filter the visible, but still emit UV too. The "blacklight" filter cuts the visible bluey-white while still transmitting the UV, thereby maximising the visibilty of fluorescent effects.
« Last Edit: 24/10/2007 18:08:22 by techmind »
 

The Naked Scientists Forum

Black light, what's all that about?
« Reply #6 on: 24/10/2007 18:05:46 »

 

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