Engelmann tried to show that photosynthesis and oxygen production are dependent on the wavelength of the light. For his experiment he split light into its individual color components with a prism. Next, he illuminated a filamentous green alga with the individual light colors. To measure the rate of photosynthesis, he added aerotactic bacteria that moved to the oxygen regions. He could see that the density of bacteria was highest in red and blue light, so the photosynthesis rate had to be highest at these wavelengths. Furthermore, he could determine that photosynthesis takes place in chloroplasts: The illumination of the chloroplasts attracted bacteria, while the illumination of other cell components failed in attracting them.
Explanation for children:
Mr. Engelmann discovered that photosynthesis is dependent on the wavelength of light. Plants do not like all kinds of light, but only blue or red light, for example. For his experiment he illuminated an alga with different colors. To measure the rate of photosynthesis, he used very special bacteria that migrate to places where the air is cleanest. He could quickly see that most of the bacteria were in the red and blue light. So that's where the air had to be the cleanest and the rate of photosynthesis had to be very high.