an action spectrum plots the rate of photosynthesis at various wavelengths of visible light, and it shows that blue light with a wavelength of about 490 nm is effective in driving photosynthesis. based on this information and the absorption spectra shown at left, what role may chlorophyll b and carotenoids play in photosynthesis? an action spectrum plots the rate of photosynthesis at various wavelengths of visible light, and it shows that blue light with a wavelength of about 490 nm is effective in driving photosynthesis. based on this information and the absorption spectra shown at left, what role may chlorophyll b and carotenoids play in photosynthesis?