Photosynthesis is most efficient at light wavelengths between 400 and 500 nanometers and 600 to 700 nanometers. The green pigment, chlorophyll, restricts the efficiency of photosynthesis.
Visible light falls between 400 and 700 nanometers. A pigment is a substance that reflects some wavelengths of visible light while absorbing others; the color of the pigment corresponds to the wavelength of light it reflects. Chlorophyll a, the primary pigment of most plants, appears green because it reflects light between wavelengths of 500 and 600 nanometers, a range that appears green to the human eye. This means that there are restrictions on the efficiency of photosynthesis such that photosynthesis is much less efficient in the green range of visible light. Light absorption by chlorophyll a and thus, the efficiency of photosynthesis, peak at about 450 nanometers and 650 nanometers. These wavelengths correspond to light that appears violet-blue and red to the human eye.
Photosynthesis still occurs, albeit less efficiently, between 500 and 600 nanometers. This is because chlorophyll a is not the only plant pigment; accessory pigments also provide extra absorption. Chlorophyll b is also green and absorbs approximately the same wavelengths as chlorophyll a. Carotenoids, such as xanthophylls and carotenes, help plants use some of the green wavelengths that chlorophyll cannot absorb. Yellow xanthophylls and orange carotenes are also responsible for autumn colors in deciduous trees.