Notes to a video lecture on http://www.unizor.com
Photochemistry
Photochemistry is so important that we can authoritatively say that, if not for photochemistry, there would be no life on Earth.
After such a dramatic statement, let's get into the real science.
Light is a source of energy in terms of individual photons absorbed by material. These photons excite material's electrons. This is the beginning and a necessary condition for subsequent chemical reactions.
The chemical reactions caused by this process of absorbing photons of light is photochemical reaction.
The one of the most important such photochemical reaction is photosynthesis - a process that is at the heart of everything that grows on Earth.
Nutrients a tree gets from the ground are components of this chemical reaction, but without light these components will not significantly interact with each other to produce new leaves and branches. Light supplies the energy needed for this reaction, and not just any light. Visible light spectrum is necessary for photosynthesis.
Some other sources of energy will not do. Not heating, nor some static electric field or mechanical oscillations.
Different photochemical reactions need different light to trigger them. Photosynthesis is an extremely complex photochemical reactions that produce some live materials, like leaves on a tree, from simple minerals the tree receives from the ground, carbon dioxide from the air and plain water.
We still are not in possession of real details of this process, it's one of the mysteries of Life.
Here are some other examples of photochemical reactions.
Somehow the Vitamin D is formed in a human body, when sunlight falls on a skin.
Plastic degrades under sun light, its molecules break down into smaller components and it loosens its physical qualities, like translucence.
Our vision is a photochemical reaction.
Solar cells, where light is converted into electricity, accomplish this via photochemical reactions.
Types of Photochemical Reactions
(from Wikipedia Photochemistry)
with h standing for Planck's constant, f - for light frequency, A and B are two substances getting into chemical reaction, when the photons are falling on them.
Photo-dissociation:
AB + h·f → A + B
Photo-rearrangements:
A + h·f → B
Photo-addition:
A + B + h·f → AB + C
Photo-substitution:
A + BC + h·f → AB + C
Photo-redox:
A + B + h·f → A− + B+
All the above reactions require the presence of photons (referenced as h·f above) to actually happen.
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