Emerson observed red drop in wavelength

Emerson observed a red drop in wavelength 680 - 770 nm.
The Emerson effect is the increase in the rate of photosynthesis after chloroplasts are exposed to the light of wavelength 680 nm (deep red spectrum) and more than 680 nm (far red spectrum). When simultaneously exposed to the light of both wavelengths, the rate of photosynthesis is far higher than the sum of the red light and far red light photosynthesis rates.