Emerson and Lewis worked on Chlorella and calculated the quantum yield for different wavelengths. Emerson observed that rate of photosynthesis declines in the red region of the spectrum. This decline in photosynthesis is called “Red drop”. It was observed that the quantum yield falls when the light of wavelengths more than 680 or 690 nm are supplied. As the PS-II P₆₈₀ is driven by red light, so it remains inactive during red drop.