ELECTROMAGNETIC WAVES (EM WAVES)

According to this theory, the energy is transmitted from one body to another in the form of waves and these waves travel in the space with the same speed as light (3 × 10⁸ m/s). These waves are known as Electro magnetic waves or radiant energy. Ex : Radio waves, micro waves, Infra red rays, visible rays, ultraviolet rays, X–rays, gamma rays.

The radiant energy do not need any medium for propogation.
The radiant energy have electric and magnetic fields and travel at right angle to these fields.
The upper most point of the wave is called crest and the lower most point is called trough.

Some of the terms employed in dealing with the waves are described below.

(1) Wavelength (λ) (Lambda) : It is defined as the distance between two nearest crest or trough. It is measured in terms of Å (Angstrom), pm (picometre), nm (nanometer), cm(centimetre), m (metre) 1Å=10^-10 m, 1pm=10^-12 m, 1nm=10^-9 m, 1cm=10^-2m

(2) Wave number ( ū ) ( nu bar) : It is the reciprocal of the wavelength, that is number of waves present in unit length.

It is measured in terms of cm^-1, m^-1 etc.

(3) Frequency (ν) (nu) : Frequency of a wave is defined as the number of waves which pass through a point in 1 s. It is measured in terms of Hertz (Hz ), s–1 or cycle/s(cps) (1 Hertz = 1 s^-1)

(4) Time period (T) : Time taken by a wave to pass through one point.

(5) Velocity (c) : Velocity of a wave is defined as distance covered by a wave in 1 second c = l /T = ln or n = c/l or c = n (s–1) × l (m) or c = nl (m s–1)
Since c is constants i.e. frequency is inversely propotional to l

(6) Amplitude (A) : The amplitude of a wave is defined as the height of crust or depth of trough.

Electromagnetic spectrum or EM spectrum :

The arrangement obtained by arranging various types of EM waves in order of their increasing frequency or decreasing wave length is called as EM SPECTRUM

Problem Based on ElectroMagnetic Spectrum

1. The Vividh Bharti station of All India Radio broadcasts on a frequency of 1368 kilo hertz. Calcu- late the wavelength of the electromagnetic waves emitted by the transmitter.

2. Calculate n in cm–1 and n of yellow radiation having a wavelength of 5800 Å