light...dark....matter

[Norman Albers]

Our talk on eye lenses clued me to the fact that I have a poor overview of light absorption. It is a complicated subject, starting with, say ordinary matter with typical atomic spacing of an Angstrom or so. Am I correct in this, speaking of solids? CRC tables had that number if I am. Visible light is maybe 5,000 A, so in a discussion where highest frequencies are ultraviolet, the light wave is still longer. Interaction changes so strongly down through different ranges. I've been through much of this but I need to feel what are the different regimes and why because I have launched a thesis of fractional photons, dark energy. I figure I had better get clear (heehee) first in the light we are accustomed to.

[BhavinB]

Not sure what your question is...can you rephrase?

[Norman Albers]

What are the different ways in which light at different frequencies interacts? Sometimes we reflect off the 'mobile electron surface". Atoms can absorb specific frequencies. Is there just no such level below visible? How does visible light warm the cat? Is there not a linear momentum exchange when atoms absorb? That's for starters, do you see? How is it things are transparent or not?

[swansont]

Visible light is maybe 5,000 A, so in a discussion where highest frequencies are ultraviolet, the light wave is still longer.

 

Shorter. Wavelength is inversely related to frequency.

[Norman Albers]

I mean still longer than an angstrom, inter-atomic spacing.

[swansont]

What are the different ways in which light at different frequencies interacts? Sometimes we reflect off the 'mobile electron surface". Atoms can absorb specific frequencies. Is there just no such level below visible? How does visible light warm the cat? Is there not a linear momentum exchange when atoms absorb? That's for starters, do you see? How is it things are transparent or not?

 

If there are transitions in the range under discussion then light will tend to be absorbed and the medium is not transparent. States certainly do exist for ranges outside the visible; molecules tend to have many vibrational and rotational states. Bulk materials tend to form bands of allowed absorptions, as the Pauli exclusion principle means the states can't be exactly the same, so they tend to "stack up" on top of each other.

 

When the light is absorbed, the atoms involved recoil and you tend to get an increase in the vibrations, which shows up as an increase in temperature. Under carefully controlled circumstances, you can do the opposite and cool the material (if it's a gas)

[Norman Albers]

So is the apparent frequency of spectral line of absorption not exactly what the atom absorbs? Also, is there inelastic scattering to warm the cat?

[insane_alien]

So is the apparent frequency of spectral line of absorption not exactly what the atom absorbs?

 

No its exactly what the atom absorbs. It'll also emit those frequencies as well if properly stimulated.

[Norman Albers]

How can this be if there is recoil? Is no energy lost there? We have a small energy impinging on a large one, if we can estimate by saying our light is 1/10 to 10 ev, and the mass-energy of the electron is 0.511 Mev. The effect wouldn't be large, but there must be some momentum kinematically, no?

[BhavinB]

There is some small amount of energy lost to vibrations.

 

And an atom will not emit only at its transition frequency, but some finite region near that frequency. This is called the 'transition linewidth'.

[swansont]

It doesn't have to re-emit a photon; the energy can go elsewhere in the system. Or it may emit one after it gets to a lower energy state.

[Norman Albers]

Thank you! I sensed there was a mess here.

[Norman Albers]

I refer readers to the continued discussion at: http://www.scienceforums.net/forums/showthread.php?t=13250