Page "Electromagnetic radiation" Paragraph 60

As frequency increases into the visible range, photons of EMR have enough energy to change the bond structure of some individual molecules.

It is not a coincidence that this happens in the " visible range ," as the mechanism of vision involves the change in bonding of a single molecule ( retinal ) which absorbs light in the rhodopsin the retina of the human eye.

Photosynthesis becomes possible in this range as well, for similar reasons, as a single molecule of chlorophyll is excited by a single photon.

Animals which detect infrared do not use such single molecule processes, but are forced to make use of small packets of water which change temperature, in an essentially thermal process that involves many photons ( see infrared sensing in snakes ).

For this reason, infrared, microwaves, and radio waves are thought to damage molecules and biological tissue only by bulk heating, not excitation from single photons of the radiation ( however, there does remain controversy about possible non-thermal biological damage from low frequency EM radiation, see below ).