Electromagnetic Radiation

Spectroscopy

The ways in which the measurements of radiation frequency (either emitted or absorbed) are made experimentally and the energy levels deduced from these comprise the spectroscopy. It gives qualitative and quantitative information.

Spectroscopy deals with interaction of electromagnetic radiation with any compound or atoms.  The interaction is measured as energy which is either absorbed or emitted by the matter in discrete amount called quanta.

Spectroscopy was originally the study of the interaction between radiation and matter as a function of wavelength (λ).

Spectrometry is the spectroscopic technique used to assess the concentration or amount of a given species. In those cases, the instrument that performs such measurements is a spectrometer or spectrograph.

Spectroscopy/spectrometry is often used in physical and analytical chemistry for the identification of substances through the spectrum emitted from or absorbed by them.

Spectroscopy/spectrometry is also heavily used in astronomy and remote sensing. Most large telescopes have spectrometers, which are used either to measure the chemical composition and physical properties of astronomical objects or to measure their velocities from the Doppler shift of their spectral lines.

There are two types of spectroscopy

1. Atomic spectroscopy

It deals the interaction of electromagnetic radiation with atoms which are most commonly in their lowest energy state called the ground state level.

DE = h g                                 here     h = Planck's constant

                                                g = Frequency of radiation    

2. Molecular spectroscopy

It deals the interaction of electromagnetic radiation with molecules which includes rotational, vibrational and election transitions of the molecule. It gives the information regarding molecular structure (molecular symmetry, bond distances and angles, and chemical properties such as electronic distribution, bond strength and intra or inter molecular processes)

Properties of electromagnetic radiation

Electromagnetic radiation is a form of energy that is transmitted through space at an enormous velocity which is equivalent to the velocity of light in the space. It requires no supporting media and can travel in vaccum. They have dual nature exhibiting both wave and particle like properties.

A. Wave properties

Wave properties of electromagnetic beam are an alternating electrical and associated magnetic force in space. They possess both the electric component and magnetic component. Both of these oscillate in plane perpendicular to each other and perpendicular to the direction of propagation of the radiation. They are coherent and plane polarized. Phase of one is related to that of other. Velocity of it is independent of frequency on vaccum (c= 3 x 10⁸ m/s). Some of the wave properties are:

1. Wavelength

It is a distance between two successive maxima on an electromagnetic wave.

Figure: Electromagnetic beam showing wavelength, l

It is denoted by ' l' lambda (Greek letter). The units of wavelength are m, cm, mm, mm, nm and A°. The beam carrying only one discrete wavelength is said to be monochromatic beam.

2. Frequency

The number of complete wavelength units passing through a given point in unit time is called frequency of radiation. It is denoted by 'g' Gamma (Greek letter).Its unit is hertz or Fresnel which means per second (s^-1). Further they can be measured in terms of KHz, MHz or GHz etc.

3. Wave number

Frequency is more fundamental than the wavelength. So it is cumbersome to use in practice because of large number of frequency in calculation. Therefore in practice, it expresses frequency in wave number. It is defined as the numbers of waves per centimeter in vaccum which is denoted by 'n' (Greek v letter) or bar in frequency.

n = 1 / l

Its unit is Kaiser or Kilokaiser (Per centimeter or cm^-1).

4. Velocity

It is defined as distance travelled by electromagnetic radiation per unit time. It is denoted by 'c'.

Velocity= wavelength x frequency

c = l x g

5. Relationship between frequency, velocity and wave number

c = l x g

or        n = g / c                                   here:   n = 1 / l

\        g = c x n  ………………….equ i

i.e. The frequency of radiation is mathematically equal to the product of velocity of radiation and its wave number.

The examples of wave properties of electromagnetic radiations are refraction, reflection etc.

B. Particle Properties

Electromagnetic radiation consists of a stream of discrete packets (particles) of pure energy called photons or quanta. They have definite energy and travel in the direction of propagation of the radiation beam with the velocity equal to that of light.

E = h . g          Where h = 6.626 x 10^-34  J. s (Planck's constant )

The example of particle property of electromagnetic radiation is photoelectric effect.

1. Relationship between wave and particle properties

We  have,        E = h . g 

            Or        E = h . c . n                 Where c is velocity and n is wave number of electromagnetic radiation

            Or        E = h . c . 1/ l             here; n = 1/ l

            \        E  µ 1/ l                     since both h and c are constant terms

Velocity is independent of frequency on vaccum. Finally the above relation shows that the energy carried by an electromagnetic beam is inversely proportional to the wavelength of the beam. It means lower the wavelength, higher the energy of the beam and vice versa. For example: The wavelength for ultraviolet rays and visible rays ranges from 190-390 nm and 400-750 nm respectively. Since the wavelength of ultraviolet rays are smaller than that of visible rays the energy carried by the UV rays are comparatively higher than that of visible rays.