Spectroscopy is that branch of physics which deals with interaction light with materials. In other words it is an analytical method for qualitative and quantitative estimation by use of light.
Light is as we all know is an electromagnetic radiation which has wave and frequency as measurable characters.
You can know more details here on the principle of spectroscopy.
Further is became a part of other means of estimation and analysis like in chromatography, elemental analysis and also identification tools.
This is mainly of 3 types which is again classified into subtypes.
The different types of spectroscopy include:
a. Based on the level of study. Here the classification is done based on the study made at atomic or molecular level. When light is allowed to pass through the substance, changes in atomic level or molecular level is observed.
1) Atomic spectroscopy: Here energy changes takes place at atomic levels. The measurement is done to study the atoms and their quantity. This is of two types as atomic absorption spectroscopy and atomic emission spectroscopy. These are referred as atomic absorption spectroscopy and flame photometry respectively.
Here the sample mostly metals and their formulations are converted to atomic forms and then measured. Ex: Sodium, calcium, magnesium related formulations.
This finds less application as the characters of most substance are depended on the molecular nature and not just atoms in it.
Also the methods are expensive, time taking and even quite tedious to perform.
2) Molecular spectroscopy: Here the energy changes occurring at the level of molecule are studied. The characters like molecular absorption. emission and vibration are studied.
The examples include, colorimetry, UV-spectroscopy, infra red, FTIR, fluorimetry etc.
This method of spectroscopy is widely used due to many applications. The methods are quick, easy and accurate in determination. Further they are easily integrated into other analytical techniques like chromatography.
B) Based on the property of either absorption or emission.
Here the principle of abortion or emission of electromagnetic radiation is taken into consideration.
1) Absorption spectroscopy: As the name suggests, here there is absorption of light by the sample. The extent of absorption and the wavelength of the absorbed light is considered. The wavelength of light absorbed tells the nature of the compound while the intensity of absorbed light tells the concentration.
2) Emission spectroscopy: Contrary to the above method, here the emitted light is measured. once the light impinges on the sample, some of it is absorbed. This absorption of light leads to transition of electrons from ground state to excited state. These excited electrons return back to ground state by release of electromagnetic radiation (light) of specific wavelength. The intensity gives the concentration while the wavelength tells the nature of the compound.
C) Based on the level of study i.e. electronic or magnetic levels.
Here the study is done based on electronic or magnetic properties of the compound. Light is an electromagnetic radiation. That is it has both electronic and magnetic properties.
Electronic spectroscopy: So when a compound is estimated without the magnetic field we call it electronic spectroscopy. In this method the substance under test is exposed to light without the influence of magnetic field.
Examples of this method are again colorimetry, UV visible spectroscopy, IR, fluorimetry etc.
Magnetic spectroscopy: Here the substance is exposed to electromagnetic radiation in presence of external magnetic field.
Examples include Nuclear magnetic resonance spectroscopy (NMR), Electron spin resonance spectroscopy (ESR).