Chlorophyll, a type of pigment, makes leaves look green because it reflects light that falls in the green part of the visible light spectrum while absorbing other wavelengths of light. Chlorophyll molecules are classified as “photoreceptors,” meaning that they capture light energy and transfer it to molecules that transform it into chemical energy. There are three main types of chlorophyll that work together to collect as much energy as possible.
The most common type of chlorophyll is chlorophyll “a,” which is common to all organisms that make their own food through the process of photosynthesis: plants, algae and certain types of bacteria. However, though it reflects green light, it doesn't absorb all the necessary high and low frequencies of light that these organisms need. Chlorophyll “b” (present in plants and algae) and chlorophyll “c” (present only in bacteria) help fill in the gaps to capture the requisite light. Because these types of chlorophyll absorb different frequencies, they also reflect different frequencies, and thus cause leaves and plants to exhibit different shades of green.
Chlorophyll isn’t the only type of pigment found in plants. When the chlorophyll in leaves begins to deteriorate with the onset of autumn, other pigment molecules become visible: “carotenoids.” They reflect red light instead of green, which is why leaves change color to adopt a reddish, orange or yellow hue.