[paulat (OP)]

Can someone please explain the difference between UVA and UVB, not just in the context of sunburn, but in terms of the physical properties of these 2 types of radiation ?

PT

[chris]

I'm resuscitating this topic to the top of the list for re-consideration by our physics gurus as the original question - the difference between UVA, UVB and UVC - was never really laid to rest.

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[Ultima]

Risk of cancer is also dependent on the time exposed to radiation. Since gamma is so penetrating it hardly ever strikes at stuff in your body whereas most of UV is absorbed by the skin, so there is a concentrated risk.

wOw the world spins?

[gsmollin]

One can Google "ultraviolet radiation" faster than starting a topic here, and get this hit: http://www.srrb.noaa.gov/UV/ which explains everything. Of course, what fun would that be?

[Ylide]

Note that only radiation which is capable of breaking chemical bonds in the body is going to directly cause cancer.  Look at the bond energies of C-H, C-O, C=O, N-H, C-N, and compare them with the energies of photons in various ranges and you'll see that the photon energies don't even get close until after the visible spectrum.  

This is why people who say that cell phones are causing cancer are morons.  They operate around the microwave area of the spectrum.  The magnetic field that close to your head may have some weird physiological effects (still being determined), but the RF energy is not going to break chemical bonds.

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[tweener]

From the NOAA link that gsmollin provided:
UVA - 315 - 400 nm (some absorbed in the atmosphere)
UVB - 280 - 315 nm (90% absorbed in the atmosphere)
UVC - 100 - 280 nm (All absorbed in the atmosphere)

The penetration of radiation into the body (or any other substance) depends dramatically on the wavelength.  The penetration of microwaves can be several cm, while light might be only a few millimeters.  UV doesn't penetrate that deeply, but it has enough energy to break chemical bonds in organic molecules (as cannibinoid said).  This is the agent that causes cancer.  

X-rays are shorter wavelength than UV, and (because of the way they interact with molecules) can travel all the way through a body, with some absorption.  They are certainly capable of causing damage to organic molecules.  That is why the X-ray tech always leaves the room after they cover the patient with a lead apron.

Gamma rays are even more energetic than X-rays and easily pass through tissue.  They also cause extensive damage when they are absorbed.  

Microwaves cannot break molecular bonds because they don't have enough energy (the wavelength is too long).  Microwaves can cause damage, but it is primarily due to localized heating of the tissues by resistive losses and to a limited extent H2O molecular resonance with the magnetic field of the waves.  There is lots of debate about microwave and radio waves causing cancer.  Obviously, a bad burn going several centimeters into the body is a nasty injury, but does it cause cancer?  The energy output from cell phones is barely detectable as heating in living tissue - quite a bit less than light exercise if I remember correctly.

----
John - The Eternal Pessimist.

[chris]

To take cannabinoid's (extremely clearly made) point, that photon energies of microwaves and beyond are insufficient to break chemical bonds, would it, however, be possible to superpose microwaves from various sources and yield a sufficiently energetic wave to enable this to happen ?

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[tweener]

quote:Originally posted by chris

To take cannabinoid's (extremely clearly made) point, that photon energies of microwaves and beyond are insufficient to break chemical bonds, would it, however, be possible to superpose microwaves from various sources and yield a sufficiently energetic wave to enable this to happen ?

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

No, because the energy in the wavefunction of the radiation is not changing.  All you would do is make more intense localized heating due to the induced current (resistive heating) and just toast the area.  The chemical bonds would certainly be affected by that, but with a fundamentally different mechanism from that due to ionizing radiation.  

The ionizing radiation in UV and higher interacts at a quantum level with the electrons (the wavelength is comparable to a molecule or even smaller), while the microwave's long wavelengths (very much larger than a molecule) just cause the loose electrons (ions) to move, which creates heat from their motion.

----
John - The Eternal Pessimist.

[Ylide]

Well put John.

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