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Spectroscopy is the study of light through light-matter interactions.
There are three main types of interactions:
- Quantized interactions in which involved molecules (Here: atoms, full or parts of molecules or crystals,..) jump from a stationary state to another stationary state. It is the most usual spectroscopy.
- Coherent interactions in which the involved molecules are excited to a non-stationary state during the interaction and return to their initial state. A detectable effect requires the coherence, that is the relations between the molecular parameters allowing the interaction (oscillating dipoles...) and the local electromagnetic fields are the same for all molecules.
- Mixed interactions: coherent and involving transitions of some molecules; example : He-Ne lasers.
Except refraction which is widely used, the coherent interactions are generally studied using lasers, so that, due to the intensity of these sources, the effects depend non-linearly on the intensity of the lasers. Some interactions require an intensity threshold.
See also 
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