Absorption Spectra Definition

Atomic spectra is referred to as the study of atoms (and their atomic ions) through their interaction with electromagnetic radiation. When a beam of light travels from one medium to another, it either bends in the direction of the normal or away from the normal. The speed of light is thus dependent on the nature of the medium through which it passes.

Absorption Spectra Characteristics

• It was observed that when a ray of white light falls on a prism it usually experience refraction twice. First, when it travels from the rarer medium (that is air) to a denser medium (that is glass) and secondly when it passes from the denser medium (that is glass) to a rarer medium (that is air).

• Lastly, a band of colours is observed, commonly termed as a spectrum, which is formed out of a ray of white light. On observing this spectrum more thoroughly, it was seen that the colour having a smaller wavelength deviates the most and vice versa.

• Therefore, a spectrum of colours is seen which ranges from the colour red to violet, where the red colour having the longest wavelength deviates the least. This kind of spectrum is often termed a continuous spectrum.

• The spectrum of the electromagnetic radiation released or absorbed by an electron during its transitions between different energy levels in an atom is termed as atomic spectra.

Types of Atomic Spectra

There are three types of atomic spectra as given below;

Emission Spectra

Absorption Spectra

Continuous Spectra

Spectra and Spectroscopy

Spectrum is broadly used in the field of optics and in many other fields. Spectrum displays a varied range of wavelengths having different frequency radiations. A rainbow is referred to as a spectrum that contains different wavelengths of light. The spectrum of light formed from the rainbow is generally referred to as VIBGYOR.

A spectroscope or Spectrograph is the device that is used to separate the radiations of different wavelengths. A spectrometer is a scientific instrument that helps to separate and measure spectral components of this physical phenomenon. Spectroscopy is the branch of science that generally deals with the study of the spectrum.

Classification of Spectra

Spectra is categorized into two types as mentioned below:

• Emission spectra

• Absorption spectra

i. Emission Spectra

The emission spectrum is commonly formed by the radiation emitted or produced by an electron in the excited molecules or atoms and this is termed as the emission spectrum. When an atom or molecule absorbs energy, then the electrons are excited to a higher energy level and when the electron falls back to its lower energy level, light is emitted or produced, which generally has the energy equivalent, to the difference between higher and the lower states energy.

Due to the availability of numerous states of energy, an electron thus can undergo many transitions, each transition gives rise to a unique wavelength that encompasses the emission spectrum. The emission spectrum is thus formed by the frequencies obtained from these emitted light.

Based on the source, the emission spectrum is further categorized into;

a) Continuous Spectrum: When the spectrum has no breaks or openings between their wavelength range then this type of spectrum is termed as a continuous spectrum. For instance; A rainbow.

b) Line Spectrum: When the spectrum has a discrete or distinct line that is atoms emit light only at specific wavelengths with dark spaces between them, then this type of spectrum is termed as a line spectrum. For instance; Hydrogen line spectrum.

Absorption Spectra

This kind of spectrum is created by the frequencies of light that is transmitted with dark bands when energy is absorbed by the electrons generally in the ground state to reach higher energy level states. This is the kind of spectrum that is produced when atoms absorb energy.

When light from any source is passed through the chemical solution, then a pattern comprising of dark lines is observed. This pattern is further analysed with the help of a spectroscope.

The dark line pattern is seen precisely in the same place where coloured lines in the emission spectrum were observed. The spectrum hence attained is termed as the absorption spectrum.

Emission spectra, unlike absorption spectrum, emit all the colours in an electromagnetic spectrum, whereas few colours in the absorption spectrum may be absent due to the redirection of absorbed photons.

Absorption Spectroscopy

Absorption spectroscopy is referred to as a spectroscopic technique that is used for measuring the absorption of radiation when it interacts with the sample.

Absorption spectroscopy is linked to the absorption spectrum because the sample thus used interacts with photons produced from the radiating field.

Applications Absorption Spectroscopy

a) Chemical Analysis: The numerical nature of absorption spectroscopy makes it a perfect choice for chemical analysis. The absorption spectrum of the different compounds can be differentiated from one another using absorption spectroscopy. This is only possible because of the specificity nature of the absorption spectrum.

b) An application of absorption spectroscopy is an infrared gas analyser that is often used for detecting the pollutants present in the air or atmosphere. It also helps to distinguish between pollutants from nitrogen, oxygen etc.

c) Remote Sensing: Absorption spectroscopy is also used for analytical purposes such as for measuring the presence of dangerous elements.

Emission Spectra vs Absorption Spectra

The key difference between emission and absorption spectra is that an emission spectrum consists of different coloured lines, however, an absorption spectrum consists of dark-coloured lines in their spectrum. Other differences between absorption and emission spectrum are mentioned below;

Emission Spectra	Absorption Spectra
The emission spectrum is formed when atoms release energy	Absorption Spectra is formed when atoms absorb energy
Generally comprises of coloured lines in the spectrum	Generally comprises of dark lines or gaps in its spectrum
The type of photons emitted from the emission spectrum is used in estimating the different kind of elements from which a substance is made as each element emits different amount of energy and generally has a unique emission level.	The wavelengths of light absorbed is used in calculating the number of substances that is present in the sample