This section amplifiesthe explanation of selective absorption and the production of spectra


The spectrum shown is that of the two kinds of chlorophyll, the constituents of all green vegetation that allows the crucial process of photosynthesis to occur; the absorption of parts of the visible spectrum in the blue and red regions that provides the energy for plants to convert CO2 and water into sugars. Without such a process there would be no life on Earth.

So, chlorophyll shows two regions of absorption with very little absorption in the region between 500-600 nanometres and it is obvious from the continuous spectrum that comes from the Sun that this region is green light. Hence, the green colours of the grass and other vegetation growing on the Earth's surface.

[1 nanometre = 10-9 metres and is used to indicate wavelengths in the visible and ultraviolet regions to keep the numbers small]

The chemistry of photosynthesis is extremely complicated and not yet quite fully understood. A simple general equation representing the beginning and ending of the process is:

xCO2 + xH2O (H2CO)x + xO2

The organic product, (H2CO)x represents the sugars and cellulose that are produced by photosynthesis and indicates why these substances are known as carbohydrates; they are a chemical combination of carbon and water: H2CO = C + H2O.

One feature of the process though is that the oxygen atoms in the product substances originate in the water molecules rather than in the CO2 molecules.

It is also worth pointing out that the reverse process; the oxidation of carbohydrates, is what happens when they are either used in metabolism of live species or when they are burned. In both cases the reactions involve the release of energy, that which originated in the radiation from the sun.