When Emerson exposed green plants to differing wavelengths of light, he noticed that at wavelengths of greater than 680 nm the efficiency of photosynthesis decreased abruptly despite the fact that this is a region of the spectrum where chlorophyll still absorbs light (chlorophyll is the green pigment in plants - it absorbs mainly the red and blue wavelengths from light). When the plants were exposed to short-wavelength light, (less than 660 nm), the efficiency also decreased.
Emerson then exposed the plants to both short and long wavelengths at the same time, causing the efficiency to increase greatly. He concluded that there must be two different photosystems involved in photosynthesis, one driven by short-wavelength light and one driven by long-wavelength (PS1 and PS2). They work together to enhance efficiency and convert the light energy to forms that can be absorbed by the plant.
The light excites the chlorophyll molecules at the reaction centre and causes an increase in energy. As the molecule becomes less excited, its energy is transported through a chain of electron carriers to the next photosystem which does much the same thing and produces energy-carrying organic molecules.