Emission and Absorption of Light

Line Spectra

We can use a prism or a diffraction grating to separate the various wavelengths in a beam of light into a spectrum. If the light source is a hot solid ( such as a light-bulb filament) or liquid, the spectrum is continuous; light of all wavelengths is present

If the source is a gas carrying an electric discharge ( as in a neon sign) or a volatile salt heated in a flame ( as when table salt is thrown into a campfire), only a few colors appear, in the form of isolated sharp parallel lines.

A spectrum of this sort is called a line spectrum. Each line corresponds to a definite wavelength and frequency.
Each element in its gaseous state has a unique set of wavelengths in its line spectrum. The spectrum of hydrogen always contains a certain set of wavelengths; sodium produces a different set, iron still another, and so on. Scientists find the use of spectra to identify elements and compounds to be an invaluable tool.

Photons and Energy Levels

Electromagnetic radiation, along with its wave nature, has properties resembling those of particles. In particular, the energy in an electromagnetic wave is always emitted and absorbed in packages called photons or quanta, with energy proportional to the frequency of the radiation.