|Why does red light affect the growth of plants?|
|Chlorophyll, the pigment in plants that makes them appear green, absorbs light principally in the violet, blue and red wavelengths of the spectrum. Most plants also have a very small amount of a pigment called phytochrome, which is particularly sensitive to red and far-red light. Red light starts a chemical reaction in the plant which causes some seeds to germinate, some plants to flower, etc. It depends on what the specific plant is.
I would suggest that you find a good botany text book for a more technical explanation. Good luck!