A travelling transverse wave of electric and magnetic fields is known as an Electromagnetic Wave. The discovery that a beam of light was actually an electromagnetic wave was made by J.C.Maxwell. Gradually, other types of electromagnetic waves were discovered.
As more discoveries were made, it was found that electromagnetic waves exist in a wide range of wavelengths, which is now called the Electromagnetic Spectrum, and visible light is only a small part of it. One of the first people to observe the spectrum was Issac Newton. He allowed sunlight coming through a small aperture to pass through a glass prism, which split it into its seven colors.
A spectrum can be pure and impure. The former is one in which light of one wavelength overlaps the adjacent wavelength. The latter is the one in which each color gives a sharp impression, as the wavelengths do not overlap. An instrument used for producing a pure spectrum is a spectrometer.
Besides the above categories, spectra can also be classified into Emission Spectra and Absorption Spectra.
An emission spectrum is produced when a light beam produced by a substance, which is excited by heating or electric discharge, is dispersed.
An emission spectra can be one of the following types:
- Continuos spectrum: It is produced when a source emits ligth with continuously varying wavelengths and gradual change in color.
- Line spectrum: It is produced when light emitted by an atom or molecule in excited state is dispersed. It appears as sharp lines on a dark background.
- Band spectrum: The spectrum appears as separate bands of varying colors.
An absorption spectrum is produced when white light is passed through an absorbing material, which absorbs certain wavelengths, before being displaced. It can be one of the following types:
- Line spectrum: The missing wavelengths are widely separated and we get sharp dark lines on a continous background, amongst some sharply defined lines.
- Band spectrum: Separate dark bands on a continous bright background, depicting missing wavelengths are observed.
A wonderful example of natural spectrum is the rainbow, which is formed when light rays passing through the atmosphere are dispersed by the water droplets present in the air.