The first action spectrum of photosynthesis was observed by

Theodor Wilhelm Engelmann, a German botanist, observed the first action spectrum of photosynthesis in 1883. He used a prism to disperse light into different wavelengths and noticed that aerobic bacteria congregated in regions of an alga exposed to red and blue-violet light, demonstrating the effectiveness of these wavelengths in driving photosynthesis. This experiment laid the foundation for understanding the role of light in photosynthesis and the importance of chlorophyll in the process.