From Simple English Wikipedia, the free encyclopedia
A scheme showing examples of photochemical reactions in nature
Photochemistry
is the study of
chemical reactions
with light. There are many natural examples. One is
photosynthesis
, how plants make
oxygen
from
carbon dioxide
and
water
. Another is making
vitamin D
in the body.
Many times reactions go with heat: as you heat
molecules
, they start to move around and so hit other molecules which can then react. What molecules need is just some
activation energy
, a little energy to start the reaction. In the case of photochemistry, this energy is given by the light. A molecule or an
atom
absorbs at a certain
wavelength
. If light is given at this wavelength then the molecule or atom becomes excited. It is then easier for it to react.
Another way in which light can help reactions go is by changing the
symmetry
of a molecule. This can be tested with the
Woodward?Hoffmann rules
. Some
pericyclic reactions
go with heat, some go only with light.
In
atmospheric chemistry
, light is very important. This is because there is much more
UV light
at the top of the
atmosphere
. This allows reactions that do not happen on the ground. For example,
ozone
can split in an
oxygen
molecule and one oxygen atom.