Some clarification please

[[Tycho?]]

Ok, for quite some time I've been trying to learn about how EM radiation is emitted/absorbed. Finding details on this is actually surprisingly difficult for some reason, but thats not why I'm here.

 

If you have a black piece of fabric out on a sunny day it will get warm, warmer than a piece of white fabric that is otherwise the same. I THINK this is why: black means most of the visible light hitting it is being absorbed, everyone knows this. So this excites the atoms/molecules. I think the warming effect is caused by the excited particles emitting radiation in the infrared spectrum, instead of the visible one. Is this correct? Or can the light directly cause movement on the molecular scale, which would cause the heat?

[BhavinB]

Yes, light can cause vibrational and rotational excitation in molecules.

 

However, perhaps the answer you're looking for is that every material can have their 'most likely absorption' energy band and their 'most likely emission' energy band in different regions. Usually, energy can be transferred in excited molecules internally through vibration and then have the remaining energy re-emitted at a different wavelength.

[[Tycho?]]

So light can cause direct heating then? Does this apply to all wavelenghts of EM radiation?

[RyanJ]

'']So light can cause direct heating then? Does this apply to all wavelenghts of EM radiation?

 

If you consider a photon as an energy transmitter then yes, every part of the EM spectrum is made of photons and the lower the wavelength the higher the energy they contain.

 

Photons give energy too the electrons of the atom as they are absorbed and so, they do have a heating effect on the atom and the substance (I think anyway).

 

For more informaiton this may be of interest: http://en.wikipedia.org/wiki/Photon

 

Cheers,

 

Ryan Jones

[BhavinB]

If you consider a photon as an energy transmitter then yes' date=' every part of the EM spectrum is made of photons and the lower the wavelength the higher the energy they contain.

 

Photons give energy too the electrons of the atom as they are absorbed and so, they do have a heating effect on the atom and the substance (I think anyway).

 

For more informaiton this may be of interest: http://en.wikipedia.org/wiki/Photon

 

Cheers,

 

Ryan Jones[/quote']

 

Giving energy to electrons in the strictest sense does not have a heating effect.

 

Heating is usually associated with vibration states (called phonons when refering to crystals). But a more complete answer is that there are internal mechanisms whereby a vibration state can be created when an electron relaxes from an excited state.

 

so.... shine light, excite electrons from level A to level C, electrons relax to level B and release energy as vibrations to the molecule, electrons relax from level B to level A (maybe cause more vibration or release light)

 

at the same time, certain frequencies of light can cause vibrations of the molecule. These are referred to as Raman modes.

[5614]

When a photon collides with an atom due to conservation of momentum we know that if the photon is absorbed then it's momentum must be transferred to the atom. So the atom will move, ie. gain kinetic energy.

 

Also if a photon is absorbed then due to conservation of energy we know that it's energy must be transferred to the atom. So the atom gains more energy.

 

Just to expand on one thing BhavinB said... phonons are what you get when you quantise vibration (kinetic) or heat energy in a lattice.