Theodore Wilhelm Engelmann (1843 - 1909), a German physiologist, showed that the light reactions of photosynthesis, which capture solar energy and convert it into chemical energy, occur within the chloroplasts and respond only to the red and blue hues of natural light. His experiments involved a modified microscope condenser that allowed him to expose parts of photosynthetically active cells of the green alga Spirogyra to a thin ray of light. His aim was to discover which components of the cell functioned as light receptors.
Energy Luminaries: Theodore Engelmann (National Science Teachers Association)
Photosynthesis: The Light Reactions (Massachusetts Institute of Technology)
Photosynthesis: The Dark Reactions (Massachusetts Institute of Technology)