Nature of electromagnetic radiation:

Electromagnetic radiation is the radiant energy which is transmitted through space at enormous velocities and does not require a medium for transmission and readily pass through vacuum.  As to wave mechanical model it could be visualised as oscillatory electric and magnetic fields travelling within the planes perpendicular to each other and also to the direction of propagation. These waves are characterised through a number of parameters such as, amplitude, velocity, frequency, wavelength, wavenumber, energy etc. Alternatively according to the quantum or particle model, the radiant energy is considered as stream of discrete particles or packets of energy called as quanta, whose energy is proportional to its frequency.

The wave nature of electromagnetic radiation is manifested in terms of a number of phenomena. Interference refers to the interaction of two waves as they are travelling. The conclusion of such an interaction depends on whether they interact constructively or destructively. The diffraction is a consequence of interference and refers to the phenomenon of bending of a wave round the corners of sharp barriers or by narrow openings.

When EM radiation passes by a transparent medium, its velocity is reduced because of the interaction of radiation with the constituents of matter. The extent of interaction is expressed within terms of the refractive index of the medium. The variation of refractive index of a substance with wavelength of the radiation is called as dispersion.

While a radiation passes from a given medium to the other medium it bends towards or away from the general to the interface. This is known as refraction and is due to the difference in the velocities of the radiation in the two media. A fraction of the radiation however, bounces back at an angle equivalent to the angle of incidence of the radiation and is termed as reflection.

Ordinary radiation is unpolarised radiation and that was consists of a bundle of electromagnetic waves having vibrations between an infinite series of planes perpendicular to the direction of propagation. On passing ordinary light by a polariser we could get polarised light having vibrations in a single plane.

The interaction of radiation with matter causes transitions between the quantised energy levels of the interacting species. While a radiation falls on the matter it may either be absorbed or it might excite the interacting species that then might emit a radiation or the radiation undergoes scattering as a consequence of interaction.

The absorption or emission spectra of atomic species consist of a series of discrete lines and is known as a line spectrum. Instead the molecular spectra are quite complex and the resulting spectra are commonly wide and the spectrum is known as band spectrum.

The radiations in the different regions of the EM spectrum have different energies. On interacting along with the matter these cause transitions amongst different kinds of quantized energy levels. This leads to various types of spectroscopies.