Why do artificial plant-growth lights have a light spectrum that favors the red and blue portions of the spectrum

because the red and blue regions are the optimum regions for photosynthesis.

Explanation:

Artificial plant-growth lights have a light spectrum that favors the red and blue portions of the spectrum because the red and blue regions are the optimum regions for photosynthesis.

Plants generally need photosynthesis to survive because, during the process, their food is synthesized. The process requires light energy to be absorbed by the green pigment of the plant - the chlorophyll. The light energy excites some electrons of the photosystems of the chlorophyll, leading to the release of the electron. The electron thus travels in an electron transport chain, eventually utilized to generate energy-rich molecules that are later utilized in other parts of the process to manufacture food.  

Investigations have revealed that the process of photosynthesis proceeds optimally in the red and light regions of the light spectrum. The discovery has made the manufacturers of artificial plant-growth light to engineer their products to favor more of these two light regions above the rest of the visible light spectrums.