| Passing white light through a prism separates the light into
different wavelengths, appearing as a rainbow of colors. The
order of colors is determined by the wavelength of light. For
visible light, red has the longest wavelength and violet has
the shortest wavelength. However, visible light is just a small
portion of the electromagnetic spectrum. Energy is inversely
porportional to the wavelength longer wavelengths have
less energy than shorter wavelengths. For example, UV light
has a shorter wavelength and more energy than visible light.
A pigment is any substance that absorbs light.
The color of the pigment comes from the wavelengths of light
that are reflected, or in other words, those wavelengths not
absorbed. Chlorophyll, the green pigment common to all photosynthetic
cells, absorbs all wavelengths of visible light except green,
which it reflects. This is why plants appear green to us.
Black pigments absorb all wavelengths of visible light that
strike them. White pigments reflect most of the wavelengths
Each pigment has a characteristic absorption spectrum describing
how it absorbs or reflects different wavelengths of light.
Wavelengths absorbed by chlorophyll and other photosynthetic
pigments generate electrons to power photosynthesis. All photosynthetic
organisms have chlorophyll a which absorbs violet-blue
and reddish orange-red wavelengths. Chlorophyll a reflects
green and yellow-green wavelengths.
Accessory photosynthetic pigments, including chlorophyll
b and beta-carotene, absorb energy that chlorophyll
a does not absorb. Chlorophyll only triggers a chemical
reaction when it is associated with proteins embedded in a
membrane, such as in thylakoid membranes of the chloroplast
or membrane infoldings found in photosynthetic prokaryotes.