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

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Engelmann used a prism to split light into its spectral components and then illuminated a green alga, Cladophora placed in a suspension of aerobic bacteria (Azotobacter). The bacteria were used to detect the sites of oxygen evolution. He observed that bacteria mainly accumulated in the region of blue and red light of the split spectrum, thus giving the first action spectrum of photosynthesis.