The term
Rhodopsin comes from rhodo- = rose or rose-red, like in
rhododendron, + opsin =
sight. It is named for the color of the
rod visual
pigment, which dominates most
retinas. This makes the name '
visual purple' rather confusing. I guess '
porphyropsin' didn't have the same ring to it.
In complete darkness, rods generate electrical events that are indistinguishable from the effect of photoisomerization found in an absorbed photon (which were first observed in toad-rods, one event every 50s).
This rate of events increases directly with the temperature, which suggests that they are due to photoisomerization of the chromophore of a rhodopsin molecule as a result of thermal agitation. Consequently, vigorous molecular collisions can occasionally flip the 11-cis isomer (mentioned in Simulacron3's above w/u) to the all-trans form. This implies that high-energy photons, thrown off by the sun, supply the Earth with not only energy that can be used a fuel but with information (rare energy values) that can be used to see it.
In humans these dark events have been estimated to be 1 event per rod every 160s, which is termed the eigengrau (a German word for 'intrinsic grey' -- there are many interesting physiological and psychological studies done on this concept itself-- including a study about LSD hallucinations and flashbacks), or the psychophysically determined limit of visual performance.
Most of the photon's energy is required for the photoisomerization to occur; this neccessary because a chromophore (lies within the opsin, which derives from chromo- = color or pigment + -phore = producer, and is a term applied to the light-catching portion of any molecule) is always taking up and throwing off small amounts of energy as a result of molecular collisions.
So, if only a small amount of energy were required to convert the chromophore to the all-trans form, then these collisions could flip the 11-cis isomer to all-trans and therefore activate the rhodopsin molecule. Then rods and cones would be unreliable because they could not distinguish light from heat.
It takes the energy of a photon in the visible range of the spectrum or higher to isomerize the chromphore. Most of the energy of the photon is taken up by the chromophore and then imparted to the entire rhodopsin molecule. This boosts the rhodopsin molecule to an activated form, and then return to its normal form, without needing additional energy.