Why does gas emit light, when light is shone on it?

[sharp910sh]

For example when the gas in the air absorbs light of a specific wavelength if emits light, in the direction of the electric field. why does this happen? what is the relationship between the wavelength of the light and the size of the gas molecule, for this to occur?

 

[mgb_phys]

Molecules absorb radiation if they have something that can change if that amount energy is applied to it.
In a gas molecule there are two things than can absorb energy.
The electrons in the atoms can absorb energy and be raised to a higher level, and emit energy when the fall from that level back to a lower one. The correct energy to do this is the same as visible light, so atoms absorb and emitvisible light. This is where the light from LED, lasers, flourescent bulbs etc comes from.
The bond between the molecules can also absrob light, but it takes much less energy to shake this bond, because the bon d between atoms is much weaker. This energy corresponds to infrared light and is why molecules are important in the greenhouse effect.

 

[sharp910sh]

Molecules absorb radiation if they have something that can change if that amount energy is applied to it.
In a gas molecule there are two things than can absorb energy.
The electrons in the atoms can absorb energy and be raised to a higher level, and emit energy when the fall from that level back to a lower one. The correct energy to do this is the same as visible light, so atoms absorb and emitvisible light. This is where the light from LED, lasers, flourescent bulbs etc comes from.
The bond between the molecules can also absrob light, but it takes much less energy to shake this bond, because the bon d between atoms is much weaker. This energy corresponds to infrared light and is why molecules are important in the greenhouse effect.

So to work out the energy of the light i use e=hf

then i can find the energy needed for an electron to jump up to the next energy level, which will emit an photon, so if the energy of the light is greater than the energy required for the electron to jump to the energy level, then it will emit light(photon). correct?

 

[mgb_phys]

Not quite
The energy of the photon has to be exactly correct to match the energy needed to promote the electrons, that is why atoms only emit and adsorb discrete wavelengths.
See http://en.wikipedia.org/wiki/Photon for a good description.

 

[sharp910sh]

ah i see. so if the energy hf is greater and smaller then e= hc/wavelength then nothing happens, but if it equals to the difference between two energy levels, then the electron can "jump" to the next energy level (not always the next depending on the energy it recieves, then when it falls back it emits a photon.