Posted: Sep 05, 2012 1:49 pm
Adco, MOST photons are certainly invisible to us. It's only those with energies corresponding to wavelengths in the region of about 400nm to less than about 800nm that are actually visible to us. That is a rather narrow range, corresponding to about 1.5 to about 3eV. EM radiation covers a hugely greater range than that, such as wavelenths up to Km and photon energies of many MeV. At photon energies of less than about 0.5MeV, they cannot interact with each other, but as soon as that threshold is exceeded, electon-positron pairs can be created by the collision of two photons, so they do interact at these high energies. Obviously, these are gamma rays, and we don't want to interact with them, and certainky don't see them.