pho·to·dy·nam·ics 
(fō′tō-dī-nămĭks)
(fō′tō-dī-nămĭks)n. (used with a sing. verb)
1. The science that deals with the effects of light on molecules, especially the activating effects of light on molecules that are inserted into the body to treat disease.
2. The behavior of a substance or chemical when exposed to light.