Beyond wavelengths of 680 nm i.e., far-red region, there is a decrease in photosynthetic yield compared to the red region of electromagnetic spectrum. This decrease is known as the Red drop. Red drop occurs because of the decrease in quantum yield. Quantum yield is the number of oxygen molecules released per light quanta absorbed. Red drop effect was proposed by Emerson and Lewis. They performed their experiment on chlorella plant.
Emerson Effect was observed by Robert Emerson in 1957. Emerson effect shows that photosynthesis proceeding very slowly when the light of 700 nm wavelength is used can be increased by illuminating with light of shorter wavelength i.e., 650 nm. So, Emerson effect is related to increase in photosynthesis when lights of two different wavelengths are provided together. Emerson effect is also known as Enhancement effect.
From red drop and enhancement experiments in the light reaction of photosynthesis, it was concluded that two photosystems are present which work at different wavelengths. It is evident of the presence of two photosystems i.e. Photosystems I and II.