Since the energy levels of matter are quantized, only light of energy that can cause transitions from one existing energy level to another will be absorbed.
We describe the energy change in the absorber as a transition or an excitation from a lower energy level to a higher energy level.
The amount of energy carried by a light photon depends on its wavelength. The shorter the wavelength, the higher the energy:
- ultraviolet or visible light absorption promotes electrons to higher orbitals or sublevels
- infrared light excites vibrations of molecules
- microwave wavelengths excite rotation of molecules
Absorption spectra are the means by which the chemical composition of our sun and other stars was discovered. The dark lines in the spectrum below correspond to elements present in the sun's atmosphere that absorb specific wavelengths of light.