**Electromagnetic radiations(EMR)** : Charged particles on moving with acceleration produce alternating electrical and magnetic fields which transmit in the form of waves and are called electromagnetic radiations or electromagnetic waves. James Maxwell was the first scientist who described EMR in 1870.

**Properties of EMR** : The properties of electromagnetic radiations are as follows :

1. The electric and magnetic fields generated by oscillating charged particles are perpendicular to each other and also perpendicular to the direction of propagation.

2. EMR do not require any medium to travel i.e. they can travel even in vacuum.

3. Electromagnetic radiations constitute spectrum which has been divided into different regions and Each EMR lies in a particular region.

4. Electromagnetic radiations are expressed by the help of their characteristic properties as follows :

● **Wavelength** : The distance between two consecutive crests and troughs is called the wavelength. It’s denoted by λ(Lambda) and its unit is ‘m’.

● **Frequency : **The number of waves passing through a point per second is called frequency. It generally denoted by **ν**. Its unit is Hz or s^{–}

^{1}.

● **Wave number : **The number of wavelengths per unit of length is called the wave number. It is denoted by **⊽**.The unit of wave number is m^{-1}.

**Key point : **If the velocity of light is **c, **wavelength is **λ** and frequency is **ν**; then **c = νλ**

**Electromagnetic spectrum : **When different types of electromagnetic radiations are arranged according to their decreasing frequencies or increasing wavelengths, then the arrangement is called the electromagnetic spectrum.