Absorption spectrum

Absorption Spectrum is a unique spectrum of light, consisting of light waves at frequencies that an atom or molecule can absorb. The absorption spectrum is primarily used in analytical chemistry for identifying a substance in a sample and in physics for studying the interactions of light with matter.

Overview

An absorption spectrum occurs when light passes through a cold, dilute gas and atoms in the gas absorb at characteristic frequencies; since the re-emitted light is unlikely to be emitted in the same direction as the absorbed photon, this gives rise to dark lines (absence of light) in the spectrum. This is in contrast to the emission spectrum, which is bright lines or light emitted from hot gas under high pressure.

Formation

The formation of an absorption spectrum is based on the quantum mechanical principle that electrons in atoms and molecules can only exist at certain energy levels. When a photon with the right amount of energy comes along, the electron absorbs the energy and moves to a higher energy level. This absorbed photon creates a gap or a dark line in the spectrum, which is unique to the atom or molecule that absorbed it.

Types

There are different types of absorption spectra. These include continuous absorption spectrum, line absorption spectrum, and band absorption spectrum. Each type of absorption spectrum has its own unique characteristics and applications.

Continuous Absorption Spectrum

A continuous absorption spectrum is one in which the absorption starts at a certain wavelength and then continuously increases with decreasing wavelength. This type of spectrum is typically observed in dense gases or solids.

Line Absorption Spectrum

A line absorption spectrum is one in which absorption takes place at specific frequencies and is characteristic of the gas absorbing the radiation. This type of spectrum is typically observed in thin gases.

Band Absorption Spectrum

A band absorption spectrum is one in which a group of lines so closely spaced that they appear as a band. This type of spectrum is typically observed in molecules.

Applications

Absorption spectra are used in various fields such as analytical chemistry, astrophysics, and environmental science. In analytical chemistry, they are used to identify the presence of a substance in a sample. In astrophysics, they are used to identify the elements present in a star or galaxy. In environmental science, they are used to measure the concentrations of gases in the atmosphere.

See Also

• Emission spectrum
• Spectroscopy
• Quantum mechanics