What makes conjugated hydrocarbons responsive to visible light more than other wavelengths of EM radiation? There seem to be different "classes" of radiation. Infrared is absorbed by (supposedly) vibrations in molecular group types, which is weird and I don't really understand it, even though I am convinced that it's useful for organic chemistry analyses since I've used it. Why is it that infrared light gets absorbed characteristically by asymmetrical electron bonds? And visible light seems to be absorbed characteristically by conjugated systems, and ionizing radiation seems to be absorbed by basically everything. I understand the idea that "photons are causing electrons to bump up a layer," but what does that mean in these contexts? Why is visible light causing that response from conjugated hydrocarbons, like the ones in rhodopsin? 