One scientist cultured Cladophora in a suspension of Azotobacter and illuminated the culture by splitting light through a prism. He observed that bacteria accumulated mainly in the region of:

This describes T.W. Engelmann's experiment. He used aerobic bacteria (Azotobacter) to detect sites of $O_2$ evolution. Since photosynthesis is most efficient in blue and red wavelengths, more oxygen was produced there, causing the bacteria to accumulate in those regions. This provided the first action spectrum of photosynthesis.