The area of chemistry and physics associated with the emission and absorption of light by atoms and molecules is called spectroscopy . The light emitted and absorbed by atoms and molecules has characteristic wavelengths or frequencies, and spectroscopic measurement of these can tell us a great deal about the energies associated with a particular atom or a particular chemical bond in a molecule since these frequencies are directly related to energy changes that take place within the molecule or in its interactions with other molecules. The wavelength and the frequency of electromagnetic radiation, or light, are simply two two different ways of measuring the same property.
Light travels only slightly slower in other media.
The visible region, which is the region of the electromagnetic spectrum visible to the average human eye, extends from about 200 nm to 900 nm. Other living organisms can detect light over slightly different regions. These are the extreme limits which the human eye can detect. Most of the sunlight available at ground level lies between 400 nm and 700 nm, because our atmosphere absorbs strongly at both longer and shorter wavelengths. The human eye is most sensitive in this region.