Introduction

Three main spectrum types are studied; continuous, line and absorption spectra. A continuous spectrum (continuous distribution of wavelengths) results from thermal radiation (also called blackbody radiation) such as from an incandescent light bulb. An absorption spectrum results from passing light through some material with the material absorbing parts of the spectrum. A line or emission spectrum results from when a specific free atom or molecule is excited to emit light, such as fluorescent or neon lights. The study of spectra emitted or absorbed by atoms and molecules forms a branch of physics called spectroscopy. Spectra provide insight into atomic/molecular structure as well as providing a mechanism for identifying some material.

An emission spectra results from an electronic transition in an atom arises when an electron in a high energy state drops down to a lower energy state and emits the excess energy as light. The energy difference between the initial (E₂) and final (E₁) electron energy states is equal to the energy (DE) carried off by the light:

D E_{carried off by light} = E₂ - E₁      (1)

The energy of the light defines its wavelength l through the relation:

Here h is Planck's constant and c is the speed of light.

Light that is observed from these energy transitions has discrete, well define wavelengths (in comparison to light from a thermal source which has a continuous spectrum). This illustrates the quantum nature of matter where the electron energy levels of an atom may only take on certain discrete (quantized) values.

The wavelengths that are emitted by an atom or molecule form a unique fingerprint of that atom or molecule and are used to identify it. The actual energy levels of the atom or molecule are usually very complex to determine, often done as a combination of experiment and approximations.