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Name: John E. B.
Status: educator
Age: 60s
Location: N/A
Country: N/A
Date: 11/26/2004


Question:
Since ultraviolet light has relatively short wavelength compared to infrared, and shorter wavelengths are supposed to have more penetrating ability than longer ones, why is it that ultraviolet light does not go through glass as well as does infrared?


Replies:
Your assumption that "shorter wavelengths are more penetrating longer ones" is not universally true. Every material has a specific characteristic range of electromagnetic radiation to which it is transparent, partially opaque, or almost totally opaque. The atmosphere for example is almost entirely transparent to visible light, absorbs part of the incident infrared from the Sun, and part of the incident ultraviolet (called UV-A and UV-B) that causes sunburn. The physics of the transmission of electromagnetic radiation can get pretty complicated. Glass that is surface treated can be very transparent in the visible part of the spectrum, but highly reflective in the infrared. Such treated glass is used as insulators in buildings.

Vince Calder


John-

Let me try to give you more perspective.

To say that "shorter wavelengths have more penetrating power" across the whole E-M wave spectrum is an oversimplification. It is only true in the high-energy range, past ultraviolet, called X-rays and gamma rays. In that range it is true because each photon has higher energy than the chemical bonds it breaks or the bound electrons it frees in the process of losing energy to the matter it is passing through. In that range matter almost rarely absorbs a whole photon in one absorption event, it just nibbles the photon down to lower and lower energies. In that range, it takes more nibbles to reach a low energy where the remaining photon is finally absorbed, so "the photon", though changed, penetrates farther.

In long wavelength ranges, shorter wavelength means a better match to the electronic resonances or energy levels of matter, so ultraviolet is actually the _least_ penetrating wavelength range in the whole electromagnetic spectrum. Especially "VUV" (Vacuum UltraViolet), wavelengths of 0.2 micron (photon=6eV) to maybe 0.002 micron (photon=600eV)

Most of the space in matter, as these waves try to penetrate, is the territory of molecular-orbital electrons, outer-shell electrons with resonant absorption energies of 4-20 eV. Photons at the top end of this energy range interact rabidly with any bound electrons they pass near, losing their energy in small fractions of a micron in solid matter. They are even absorbed quickly in gasses such as air. They only survive in a vacuum, hence the name "VUV".

The deeper orbital electrons in larger atoms have higher ionization energies, up to roughly 100keV in heavy elements like lead, but these electrons are confined to small round volumes near each nucleus, so the photon has a pretty good chance of passing many without interacting. And in the energy range above 10keV, the photon's wavelength starts getting smaller than the distance between two nuclei, so it's also possible for the photon to be localized in the space between atoms, rather than being a broad wave-front immersing many atoms at once.

Think about the long-wavelength end of the spectrum, too. Long radio waves can go through miles of the best non-conductive substances, to distances of roughly 10,000 wavelengths. In that range, longer wavelengths have more penetrating power. DC magnetic fields might be considered waves of photons of sub-1Hz frequencies.

They are very penetrating if the substance does not have magnetic atoms like iron, and sometimes even when it does. The earth's polar magnetic field is an example, it penetrates a thousand miles of crust and mantle.

In the middle, there are a few different absorption bands, and windows between those absorption bands. Infra-Red waves are absorbed by the vibrational resonances of matter rather than by exciting the electrons. In between the vibrational energies and the electron-excitation energies, there is often a "window" such as the transparency of glass. Glass fiber optics can have absorption rates below 20%/mile at 1.31 or 1.55 microns wavelength.

It is difficult to find any resonant infra-red absorptions with sub-micron absorption length. The field-exclusion or skin depth of highly conducting metals is about the only thing which can block IR waves that quickly. And that is largely reflection not absorption. Infra-red absorption peaks in non-conductive materials have attenuation lengths more like 100 microns.

Jim Swenson



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