Red Drop or Emerson's First Effect

Emerson investigated Chlorella using monochromatic light - a single wavelength at a time - and measured the resulting quantum yield of oxygen evolution across a range of wavelengths, aiming to find exactly which wavelength produced the maximum photochemical yield of oxygen. He found that the yield stayed essentially constant across the 600 to 680 nm range, but then fell sharply and unexpectedly once the wavelength moved beyond 680 nm, into the deep red region - even though chlorophyll a still absorbs meaningfully in that region. This sudden, otherwise unexplained collapse in photosynthetic yield at wavelengths just beyond the red absorption peak is called the red drop, or Emerson's first effect, and it strongly suggested that light absorbed at these longest usable wavelengths could not, by itself, d …