Emerson & Lewis (1943) measured quantum yield at different wavelengths of light. Quantum yield can be defined as a number of O2 molecules released per quantum of light absorbed. A sudden drop in the rate of photosynthesis was noticed at 680 mµ (red region). This sudden fall in the photosynthesis yield beyond the red region of the spectrum is called red drop.
Emerson et al further noticed that photosynthetic rate can be restored if a simultaneously shorter wavelength is provided. This simultaneously giving of shorter & longer wavelengths gave photosynthetic rate higher than the total rate from the beams separately. This photosynthetic enhancement is referred as Emerson enhancement effect or Emerson effect.
Fig: Red drop