Photosynthetic enhancement with flashing light was first observed by

Photosynthetic enhancement with flashing light was first observed by Emerson and Arnold.
Emerson and William Arnold found that "the yield per flash reached a maximum when just 1 out of 2500 chlorophylls absorbed a quantum".
Next, in 1939, Emerson demonstrated that between 8 and 12 quanta of light were needed to produce one molecule of oxygen. Thus, changes in flashing light lead to enhancement in the photosynthesis process.