The Emerson effect is the increase in the rate of photosynthesis after chloroplasts are exposed to light of wavelength 670 nm (red light) and 700 nm (far red light). When simultaneously exposed to light of both wavelengths, the rate of photosynthesis is far higher than the sum of the red light and far red light photosynthesis rates. Wavelengths beyond 700nm are apparently of insufficient energy to drive any part of photosynthesis. So a huge drop in efficiency has been noticed at 700nm. This phenomenon is called as Red drop effect. In other words there is a sharp decrease in quantum yield at wavelengths greater than 680nm. This decrease in quantum yield takes place in the red part of the spectrum. The number of oxygen molecules released per light quanta absorbed is called as quantum yield of photosynthesis. This effect was first of all noticed by Robert Emerson.