# Ultraviolet Radiation and Heat



## Umbra (Oct 30, 2011)

Greetings to everyone from CandlePowerForums! I am very glad that I found this forum since there are a lot of knowledgeable individuals here on the field of light. 

If anyone is willing to help me clarify some conceptuals before I start work on a radiation experiment it would be greatly appreciated!

The relationship that will be explored in this experiment is the effect of density of a substance on its transmittance of UV light (or just radiation in general if the design doesn't pan out).
What I am thinking of doing is placing a Thermos bottle containing 100mL of water under an ultraviolet lamp with a thin sheet of cloth (could be another substance) covering the top of the thermos bottle. The temperature of the water is then measured with a temperature probe after an hour (tentative) of exposure, and this temperature difference will be used in conjunction with the specific heat capacity of water to judge how much ultraviolet light was let through the cover.

The density of the substance is then manipulated by manipulating the mass of the cover (such as compacting cotton) while controlling the thickness of it. The area of the substance will be the same through all trials as controlled by the area of the opening of the Thermos bottle.

Some preliminary testing with the Thermos bottle and a 25W incandescent lamp gives the result of a temperature change from 22.1 to 76.5 degrees Celsius (I'm Canadian) after 45 minutes of exposure. After setting it to cool for an hour, the temperature dropped to 67.2 degrees Celcius, which means that the heat retention is not a problem. 

I am wondering if this is conceptually sound on the basis that absorption of ultraviolet light by water will induce a temperature change? (Sorry I don't have access to any ultraviolet lamps at the moment x_x)

Some research and logic brought conflicting responses:
-UV light is too energetic to create a vibration in the water molecules (average kinetic energy of molecules is temperature), instead it breaks chemical bonds by bombarding electrons
-UV light is cool, therefore we cannot feel it (although I feel this is an inadequate explanation
-UV light is a form of electromagnetic radiation and therefore like all radiation should produce heat when absorbed by a molecule (logic)

If ultraviolet light will not increase the temperature, then I am wondering if this experiment will be valid if I consider other forms of radiation (such as visible light and infrared light) and then relate it back to ultraviolet radiation by saying similar results will be produced?


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## Ken_McE (Oct 30, 2011)

If you send enegry into a container, the container will heat up.


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## Umbra (Oct 31, 2011)

Thanks for the response; will it heat up the same if the lamp was incandescent instead (with same power output)?


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## Ken_McE (Nov 1, 2011)

There are only four things that can happen when electromagnetic radiation strikes a surface. It can pass through, reflect, diffuse, or absorb. A thermos bottle will transmit none of the light, so we can rule that out. It sounds like none of the energy will be allowed to reflect back out of the container, so we can consider that it will all be absorbed.

Energy can do various things when it interacts with matter, but in the end it all becomes heat. It doesn't matter what kind of energy you put into the bottle, it only matters how much. One watt of radio waves, terahertz waves, microwaves, red light, blue light, UV light, whatever, if it gets trapped in there it will all fade into being one watts worth of heat.

The above notwithstanding, I think you should do the experiment and see what happens.


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## Canuke (Nov 6, 2011)

I think there's a fifth option: fluorescence/downconversion. That's the process by which all that light degrades to heat, and why the initial wavelength of the energy source ultimately doesn't matter (assuming a perfectly closed system).


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