Explain light activation of pigments (absorption of light by pigments)?
A pigment is a substance that imparts color by absorbing some frequencies of visible light but not others. For instance, there are a lot of substances that absorb ultraviolet light into the visible spectrum, in other words they also absorb plain violet light. Since they absorb violet light but reflect back the rest of light, they appear yellow. Purple pigments, on the other hand, are quite rare because they absorb purple light (which has the highest energy of visible light) and reflect back everything else. When anything absorbs a photon of electromagnetic radiation (light, x-rays, ultraviolet, infrared, microwaves, gamma rays, radio waves), it is activated which means that it takes the energy of the photon and goes to an energy state that is higher by the same amount of energy that was in the photon. At the molecular level, energy is quantized, meaning its restricted to particular states. For instance, vibrational energy corresponds to infrared light: there are only certain ways, called modes, that a molecule can vibrate in, if it can't vibrate in an appropriate mode, it can't absorb the infrared radiation that corresponds to being promoted to that mode. That's why substances can be transparent. At the higher energy state, the substance might be able to participate in chemical reactions that it would not be able to participate in in a lower state. That's usually what is meant by light activation. So a pigment that absorbs visible or UV light might become activated and react with something or react in ways that it wouldn't be able to in the dark.
