Hi, please could someone help as I have got myself desperately confused with how radiation interacts with matter largely because of conflicting sources on the internet. Apparently there are 2 main types of interaction of radiation with matter; atomic/molecular vibrations and electron transitions. There are a few things I am confused with: 1) Most sources I have read say a photon frequency of infrared or lower mainly interacts via atomic/molecular vibration although some sources say infrared can cause energy level transitions but most say infrared does not have enough energy to cause a discrete transition. Could someone clarify/confirm this? 2) If humans absorb visible light photons why can't they emit visible light (as we emit in the infrared). This seems stupid and clearly we don't but if we absorb visible light photons which cause discrete electron transitions then couldn't the electron jump straight back down to the ground state and emit a visible photon. What am I missing here? 3) I'm a bit confused with the vibrational absorption of radiation. It seems to suggests that you need a change in the dipole moment to absorb say, an infrared photon and that it depends on the natural frequency vibration of the molecule in question. But surely ALL molecules must interact not just ones that have a matching natural frequency because all objects emit infrared otherwise they would be at absolute zero? Many thanks for any help anyone gives!!!