## Electromagnetic waves

1) These are transverse waves in which there are sinusoidal variations of electric and magnetic fields at right angles to each other as well as at right angles to the direction of wave propagation.

2) Both these fields vary with time and space and have the same frequency.

3) These waves can travel through vacuum.
4) Examples of the electromagnetic waves are radio waves, micro waves, infrared rays, light waves, ultraviolet rays, X-rays and gama rays.

### Intensity of electromagnetic wave

The energy of electromagnetic wave crossing per unit time per unit area perpendicular to the direction of propagation of wave is called the intensity of electromagnetic wave.

### Momentum of electromagnetic wave

The electromagnetic wave during its propagation has linear momentum with it. The linear momentum carried by the portion of wave having energy U is given by p = U/c . If the electromagnetic wave incident on a material surface is completely absorbed, it will deliver energy U and momentum p= U/c; If the incident wave is totally reflected from the surface, the momentum delivered to the surface = U/c-(-U/c)= 2U/c. Due to which the electromagnetic waves incident on a surface exert a force on the surface.

### Radiant flux of electromagnetic wave

According to Maxwell, the acceleration charge particle produces electromagnetic waves. The total radiant flux emitted at any instant is given by P = q2 a2/(6πє0c3), Where q is the charge on the particle and a is its instantaneous acceleration.

The average value of pointing vector (S) over a convenient time interval in the propagation of electromagnetic wave is known as radiant flux density. When energy of electromagnetic wave is incident on a surface, the flux density is called intensity of wave. (Denoted by I).Thus I = S.

### Production of electromagnetic waves

1) The oscillation or accelerated charge produces electromagnetic waves. This concept has been used by Hertz oscillator.

2) The electromagnetic wave is emitted when an electron orbiting in higher stationary orbit of atom jumps to one of the lower stationary orbit of that atom.
3) The V electromagnetic waves (i.e. X-rays) are also produced when fast moving electrons are suddenly stopped by the metal of high atomic number.

### Electromagnetic spectrum

The orderly distribution of electromagnetic radiation according to their wavelength or frequency is called the electromagnetic spectrum.

### Amplitude modulated range

Electromagnetic waves of frequency less than 30 MHz (or wave length 10 m or more) from amplitude modulated range.

### Frequency modulated band

The electromagnetic waves of frequencies between 80 MHz to 200 MHz from frequency modulated band. These waves are not reflected by ionosphere.

### Height of transmitting antenna

It is related with the relation d = √2hR, Where d is the radius of the circle on the surface of earth within which the transmitted signal from the transmitted antenna can be received and R is the radius of earth. Area covered =πd2 = π(2hR) Population covered = (area covered)(population density).

### Micro waves

1) These are the electromagnetic waves of wavelength of the order of a few millimeters.
2) The microwaves can be transmitted as a beam signal in a particular direction, much better than radio waves.

3) The microwaves do not spread or bend around the corners of any obstacle coming in their way.

4) Microwaves are used in radars to locate the flying objects like aero plane, jet planes etc.

### Modulation

It is a process of superimposing an A.F signal over R.F. carrier waves. Modulation can be-

a) amplitude modulation and
b) frequency modulation

### Demodulation

It is a process of separating the A.F. signal from the modulated waves.