Absorption Spectrum - Spectra and Spectroscopy, Emission Spectra and Absorption Spectra

Light is an electromagnetic wave and it has a frequency and a wavelength. This wave can travel in a vacuum at the speed of light. The entire range of the electromagnetic spectrum is given by radio waves, microwaves, infrared light, ultraviolet light, X-rays, and gamma rays. The interaction of electromagnetic waves with matter causes the molecules and atoms present in a matter to absorb energy and reach a higher energy state. They have to emit energy in the form of radiation to return to their original state as the higher energy state is unstable and this process produces absorption spectra and emission spectra.

Energy State

We know that energy is conserved. This means it can not be created or destroyed. Whenever an electron loses energy, a photon is created with the exact amount of energy lost. Whenever an electron gains energy, a photon with that amount of energy is absorbed. If electrons absorb energy, they can jump from a lower energy state to a higher energy state. On the contrary, when it transits from a higher energy state to a lower energy state, they emit energy. So, absorption or emission takes place when electrons transit from one energy level to another. We can find each possible photon’s energy using the electron energy levels.

Quantized Energy

Electrons can not exist between energy levels in an atom. The energy of an electron is quantized means it can only exist in certain specific quantities, and not between those quantities. We can use electron energy level diagrams to find all possible energies of emitted and absorbed photons. Absorption and emission spectra are evidence for the fact that electron energy levels are quantized.

What Is Spectrum?

The band of colours obtained by passing light through a prism is called a spectrum. A rainbow and a prism are two ideal examples to describe the spectrum of radiation. The instrument which we use to separate the radiations of different wavelengths is called Spectrograph or Spectroscope and the branch of science which deals with the study of the spectrum is known as spectroscopy. The spectra obtained from different substances can be classified into two types:

• Absorption Spectrum: An absorption spectrum can be defined as a spectrum obtained by transmitting electromagnetic radiation through a substance. It is the spectrum of radiation emitted by a substance that has absorbed the energy. It is produced when atoms absorb energy.

• Emission Spectrum: An emission spectrum can be defined as a spectrum of the electromagnetic radiation emitted by a substance. This spectrum is produced when atoms release energy.

When white light passes through a prism, it breaks up into seven colours. They are violet, indigo, blue, green, yellow, orange and red known as VIBGYOR. This we can see as a band of colours. The spectrum which is produced by splitting white light is continuous in the sense there are no gaps between these colours. So, we can say that the band of these seven colours is known as the spectrum. As this spectrum consists of different shades of each of these colours without any gaps, such a spectrum is called a continuous spectrum.

Absorption Spectrum

As we can see from the figure given below that light from a light source falls on a sample of gas. Then it travels through the slit and falls on the prism. So, dispersion of light takes place and then the spectrum can be seen on the detector. The spectrum obtained is the absorption spectrum which is also shown in the figure given below. Such a spectrum will have some dark lines which mean some of the wavelengths have been absorbed by the sample of gas.

When electrons jump to higher energy orbits, they will absorb energy and as there is an absorption of energy, those wavelengths can be seen as dark lines. This is called the absorption spectrum. We can say that the absorption spectrum will have dark lines corresponding to the wavelengths that have been absorbed. So, in simple words, we can say that the absorption spectrum shows the spectrum of Electromagnetic wavelengths passed through a substance, with dark lines due to the absorption of specific wavelengths.

Applications Of Atomic Absorption Spectroscopy

Following are the applications of atomic absorption spectroscopy.

• To determine metallic elements in biological materials.

• To determine calcium, magnesium, potassium and sodium in blood serum.

• To determine metallic elements in the food industry.

• It is useful in environmental studies for the detection of metallic impurities in drinking water, soil and ocean.

• To determine the concentration amount of lead present in petrol, we use atomic absorption spectroscopy.