Maximum photosynthesis occurs in red light. When a plant is given monochromatic light (single light colour), the highest absorption of blue light is seen, but the highest photosynthesis rate is seen in red light. Light is absorbed by photosystem II that is membrane-bound. It absorbs infrared and red light. On the other hand, blue light is absorbed by chlorophyll and carotenoids, but some carotenoids are not found in chloroplasts.
- Carotenoids are constituted by the carotenes and xanthophylls, where carotenes do not transfer absorbed energy effectively to chlorophyll.
- Hence, some portion of the absorbed light falling under blue is not passed to photosynthesis while all the red light is absorbed by chlorophyll and effectively used.