Trees and plants are green because of a green
pigment called chlorophyll. This pigment
absorbs red light the best, and converts
the light into energy that it uses for metabolism.
As you likely know, this pigment allows plants to
use light as a form of energy, as a part of a
process called photosynthesis.
Instead of eating food to build molecules,
plants can take light from the sun and use the
energy to convert carbon dioxide from the air into
useful molecules! However, the pigment doesn't
strongly absorb blue or green light, so plants
can't use this energy for photosynthesis as
effectively. Interestingly, we know this even from
the colour of plants! White light contains all the
colours, and plants appear green because they
absorb the red light, leaving what appears to us
as green light, to be seen by our eyes! If the
light isn't being absorbed by the plant, it can't
be used for photosynthesis!
Although I've specifically talked about
chlorophyll, plants have developed several
different pigments that absorb different colours,
but chlorophyll is certainly the most common.
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