When one bell is rung, the other will sympathetically ring at the same frequency due to transfer of energy from the struck bell. The resonance of the second bell matches the frequency of the first ringing bell and hence the second bell accepts energy from the struck bell. Photons are electromagnetic waves with a particular frequency. Unlike the bell analogy, photon absorption occurs as a quantum event, an all or none phenomenon. Without absorption, there is no energy transfer to the tissue and the tissue is left unaffected by the light.
Absorption of light provides a diagnostic role such as the spectroscopy of a tissue.
Absorption can provide a clue as to the chemical composition of a tissue, and serve as a mechanism of optical contrast during imaging. Absorption is used for both spectroscopic and imaging applications. OMLC homepage .