There is a large number of reactions due to light in organic chemistry and one of fundamental importance to mankind is the effect of light on the chlorophyll, the green coloring matter in vegetation.  No permanent change takes place in the chlorophyll, but by the action of light it enables the plant to absorb oxygen, carbon dioxide, and water and to use these to build up the complex organic substances which are found in plants.  Radiant energy or light is absorbed and converted into chemical energy.  This use of radiant energy occurs only in those parts of the plant in which chlorophyll is present, that is, in the leaves and stems.  These parts absorb the radiant energy and take carbon dioxide from the air through breathing openings.  They convert the radiant energy into chemical energy and use this energy in decomposing the carbon dioxide.  The oxygen is exhausted and the carbon enters into the structure of the plant.  The energy of plant life thus comes from radiant energy and with this aid the simple compounds, such as the carbon dioxide of the air and the phosphates and nitrates of the soil, are built into complex structures.  Thus plants are constructive and synthetic in operation.  It is interesting to note that the animal organism converts complex compounds into mechanical and heat energy.  The animal organism depends upon the synthetic work of plants, consuming as food the complex structures built by them under the action of light.  For example, plants inhale carbon dioxide, liberate the oxygen, and store the carbon in complex compounds, while the animal uses oxygen to burn up the complex compounds derived from plants and exhales carbon dioxide.  It is a beautiful cycle, which shows that ultimately all life on earth depends upon light and other radiant energy associated with it.  Contrary to most photochemical reactions, it appears that plant life utilize yellow, red, and infra-red energy more than the blue, violet, and ultra-violet.

In general, great intensities of blue light and of the closely associated rays are necessary for most photochemical reactions with which man is industrially interested.  It has been found that the white flame-arc excels other artificial light-sources in hastening the chlorination of natural gas in the production of chloroform.  One advantage of the radiation from this light-source is that it does not extend far into the ultra-violet, for the ultra-violet rays of short wave-lengths decompose some compounds.  In other words, it is necessary to choose radiation which is effective but which does not have rays associated with it that destroy the desired products of the reaction.  By the use of a shunt across the arc the light can be gradually varied over a considerable range of intensity.  Another advantage of the flame-arc in photochemistry is the ease with which the quality or spectral character of the radiant energy may be altered by varying the chemical salts used in the carbons.  For example, strontium fluoride is used in the red flame-arc whose