Radiation can only affect matter if absorbed by it. Within the broad
range of 300-1000 nm, which we call "the visible", light quanta are
energetic enough to produce excited electronic states in the atoms and
molecules that absorb them. In these states the molecules may have quite
different properties from those in their dormant condition, and
reactions that would not otherwise occur become possible. About 80 % of
the radiant energy emitted by our sun lies in this fertile band, and so
long as the sun's surface temperature is maintained at about 6000° C
this state of affairs will continue. This and the transparency of our
atmosphere and waters have allowed the generation and evolution of life.
Before life began the atmosphere probably also transmitted much of the
solar short-wave radiation, but with the rise of vegetation a new
product - oxygen - appeared and this, by a photochemical reaction in the
upper atmosphere, led to the ozone layer that now protects us from the
energetic "short-wave" quanta that once, perhaps, took part in the
generation of life-molecules. Light is an ideal sensory stimulus. It
travels in straight lines at great speed and, consequently, can be made
to form an image from which an animal can make "true", continuous and
immediate assessments of present and impending events.