## Definition of the electromagnetic radiation spectrum

The electromagnetic spectrum radiation chart can be described as a wave occurring simultaneously in the electrical field and magnetic field. Each type of radiation like radio waves, ultraviolet, infrared, visible, etc has both wave and particle properties.

Thus the particle properties can be described by quanta or photons but the wave describes by wavelength or frequency.

#### Wavelengths of the electromagnetic spectrum

The wavelength is the distance between two consecutive waves. Thus the wavelengths of the electromagnetic spectrum are express in either meters, millimeters, micrometers or nanometers.

In addition to wavelength, the radiation also expressed in frequency. Frequency is defined as the number of complete cycles per second (cps) and also called Hertz according to the name of German physicist H.R Hertz.

### Wavelength and frequency

From their definition wavelength and frequency are inversely proportional to each other.

where c = 3 × 1010 cm sec-1 = velocity of light.

de Broglie provides a direct relation between the frequency and energy of electromagnetic energy or photon.

$E&space;=&space;h\nu&space;=\frac{hc}{\lambda&space;}$

where h = Plank constant

Thus according to the relation, the higher the frequency or shorter wavelengths of radiation will be greater is its energy. Therefore X- rays are more energetic than visible light.

### Electromagnetic spectrum chart

The wavelengths of the electromagnetic spectrum flow from 400 nm (violet) to 750 nm (red).

 Region Wavelength (λ) Frequency (ν) Cosmic rays 5 × 10-5 nm Gamma rays 10-3 – 15 nm X – rays 0.01 – 15 nm Far UV 15 – 200 nm 666,667 to 50,000 cm-1 Near UV 200 – 400 nm 50,000 to 20,000 cm-1 Visible 400 – 800 nm 25,000 to 12,500 cm-1 Near IR 0.8 – 2.5 μ 15,500 to 4000 cm-1 Vibrational IR 2.5 – 25 μ 4,000 to 400 cm-1 Far IR 0.025 – 0.5 nm 400 to 200 cm-1 Microwave 0.05 – 300 nm 200 to 0.033 cm-1

Thus from the above chart visible region of electromagnetic radiation is a very small part of the entire spectrum. But the wavelengths of the visible region slightly higher than IR-spectrum and slightly lower than the UV-spectrum.

### Types of the spectrum of electromagnetic waves

The spectra of substances radiate variously wavelengths. If the energy of light does not match, then the light is not absorbed by the substances. Thus the spectra provide the most important physical properties of inorganic and organic compounds.

Therefore spectroscopy is an important tool for structure determination. Thus absorbed energy of electron brings the substances in different kinds of excitation.

1. UV and visible light bring the valence electrons from lower energy levels to higher energy. Thus UV-visible light changes in electronic energy levels to forms the atomic spectra in the molecules.
2. IR spectroscopy causes vibrational excitation among the molecules. Thus IR spectroscopy changes the vibrational and rotational movements of the molecules.
3. But microwave forms NMR spectroscopy in the molecules. It affects rotation around the chemical bond of the molecules.