Using electromagnetic radiation as a probe to obtain information about atoms and molecules that are too small to see.
Electromagnetic radiation is propagated at the speed of light through a vacuum as an oscillating wave...
Rotational transitions are of little use to the spectroscopist. Rotational levels are quantized, and absorption of IR by gases yields line spectra.
However, in liquids or solids, these lines broaden into a continuum due to molecular collisions and other interactions..