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Name: Ghazal
Status: student
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Question:
Why does UVA penetrate deeper than UVB ? I thought that the ray that has a shorter wavelength will have more energy.


Replies:
Hi Ghazal,

Light (and electromagnetic radiation in general) acts differently than mass. For mass, you might expect a higher energy, more-massive projectile to penetrate more deeply into something than a lower-energy, less massive projectile. However, with light, penetration depends on the material it is hitting. Higher energy photons do not necessarily mean higher penetration.

Light is made up of photons, each with a characteristic amount of energy. The amount of energy in each photon has to meet a material that has the right kind of composition (such as a chemical bond) that can absorb the photon -- if not, the photon will not be absorbed. This explains why different objects appear different colors, or why some are transparent.

With mixtures or layered materials, one material/layer may absorb one kind of light, while another material/layer may absorb another. Thus, depending on how much of the light gets absorbed, some wavelengths might penetrate more than others.

Hope this helps,

Burr Zimmerman


Ghazal -

"the ray that has a shorter wavelength will have more energy." That is quite true. And you also seem to be thinking: "the ray with more energy will penetrate farther". That, however, is true only for X-rays and gamma-rays and charged particles, radiations with energies above 1000 electron-volts, which repeatedly lose bits of their energy as they blunder their way through matter. If you only loose about 4 eV every time you bump an atom, then the more eV's you start with, the longer it will take you to reach zero energy.

For UV light, (photons with energy from 3 to 10 eV) the opposite is usually true. For example, imagine clear glass, transparent at all wavelengths of visible light. At some distance into the UV spectrum, this glass will start absorbing the photons and become "dark". Even pure clear air will absorb light that is far enough into the UV. In fact all gases do this, when the photon energy gets high enough to match the atom's lowest excitation energy, and become able to excite an outer electron to a higher orbital.

A similar thing is true of skin. It is largely transparent, but each bio-molecule in it has some threshold energy to start absorbing UV light. And when flesh absorbs energetic light, some chemical reactions result, and some damage is done. Melanin, the brown coloring in one's skin, is the chemical least likely to cause harm when it absorbs such energy. And its absorption wavelength threshold is lower than those of the other molecules in your skin, so melanin protects them by absorbing the light before the others do.

Many brownish or yellowish substances have gradual onset of absorption in the UV. The shorter the wavelength, the faster the light is absorbed. This is probably true of melanin too. So UVB's penetration will be shallower than that of UVA, for moderately dark skin. For very light skin, the same thing is probably also true, even though the absorptions will be due to other unspecified bio-molecules.

Jim Swenson


Ghazal:

Penetrate what?
Skin? Photosynthetic material?
From the following article:

http://en.wikipedia.org/wiki/Ultraviolet

UVA has the higher wavelength/lower frequency/lower energy than UVB. Your thought that shorter wavelengths/higher frequencies/higher energies is correct.

Sincere regards,

Mike Stewart



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