The visible region, despite being the most familiar to all humans (as it is the only region that the human eyes can see), occupies a very small region of the electromagnetic spectrum, with wavelengths between 7x105 cm and 4x105 cm, which corresponds to photon energies of about 2.2 eV in the center of this range.
The visible region extends between the infrared and ultraviolet regions in the following order:
A common acronym used for the increasing wavelengths in the visible region is VIBGYOR.
The human eye is only able to swee light waves of this region. This is attributed to the fact that light waves of frequencies with absorption coefficients larger than 10 cm-1 do not reach the retina, since the water present in the constituent fluids absorbs such frequencies of light. Absorption of radiation is strong near the infrared region on account of the molecular movements. In the ultraviolet region however, absorption is caused by electronic excitations.
About half of the Sun's radiant energy is contained in this region, and is vital for the existence of life on Earth. It is necessary for many processes, such as photosynthesis and decay of organic matter. The energy stored in the organic fuels like coal and oil, is just the stored energy absorbed by the plants from the sunlight. Efforts to use this source of energy started few years ago with the attempt to convert solar energy into electric energy in the 1970s.
Today, with the help of photothermal and photovoltaic technologies, solar energy is being widely used. Whereas in photothermal devices, sunlight is used to heat some substance, photovoltaic devices directly convert solar energy into other forms with the help of solar panels, which are being used quite commonly throughout the world today.
Visible light is also made use in many other machines today such as photoelectric copying machines, laser printers, CD players, FAX machines, optical recording media, and optical disc mass-storage systems of exceedingly high bit density.