Propagation of Electromagnetic Waves in the Atmosphere

Waves help transfer energy from one medium to another medium without the flow of whole matter.

Mainly, there are 3 types of waves:

1. Mechanical waves
2. Matter waves
3. Electromagnetic waves 

Mechanical Waves 

Mechanical waves are those waves that need a medium for their propagation. They do not propagate through a vacuum. Examples include water waves and sound waves.

Further, there are two types of mechanical waves:

1. Transverse waves, and 
2. Longitudinal waves

Matter Waves

Matter waves are also called De-Broglie waves. These waves are associated with material particles; electrons, protons, neutrons, atoms, and molecules have wave-like motions. For massive bodies of waves, nature is negligible.

Electromagnetic waves

Electromagnetic waves are the form of visible light that enables us to view the world around us. There are various electromagnetic waves, such as infrared rays, microwaves, gamma rays, ultraviolet rays, etc. 

Let us understand more about electromagnetic waves.

Electromagnetic Waves

Electromagnetic waves are combined oscillations of electric and magnetic fields propagating through a vacuum. These fields are perpendicular to each other and perpendicular to the direction of energy and wave propagation, which forms a transverse wave.

When a charged particle moves or gets accelerated, there is the production of electromagnetic waves. Subsequently, these waves interact with the charged particles.

Electromagnetic waves are of 7 types, abbreviated as RMI LUX G.

R → Radio waves

M → Microwaves

I → Infrared Radiations

L → Visible Light

U → Ultraviolet Radiations

X → X-rays

G → Gamma rays

The speed of electromagnetic waves in a vacuum is:

c = 3 x 108 m/s

Electromagnetic waves propagation or transmission occurs through radio waves in the atmosphere with the help of waves transmitted via the transmitter antenna radiating electromagnetic waves. Waves travelling through space reach the other end of the antenna.

Propagation of Electromagnetic Waves

Following are the methods of propagation of electromagnetic waves:

Ground or surface wave propagation

1. In this propagation method, signal waves transmitted from the transmitter antenna to the received antenna disseminate under the strong influence of the ground mode of transmission of waves.
2. Induction of current occurs when the waves propagate on the earth’s surface. The bending of ground waves occurs when interacting with the objects on it.
3. Due to this fact, the earth absorbs the energy of the ground wave, and hence, the power decreases.
4. As the distance of the transmitting power station increases, the power of the ground wave decreases. In this propagation mode, when the power of the ground wave is lost, this property is called attenuation.
5. The rise in the frequency of the ground wave increases the attenuation of the ground wave.
6. Ground waves do not transmit signals of high-frequency waves and long-range communication.
7. Signals of high frequency are transmitted when the size of the antenna is comparable with the signal’s wavelength,

Skywave propagation

1. In this mode of propagation, the ionosphere plays an important role. Levels of the atmosphere are Troposphere, Stratosphere, Mesosphere, and Ionosphere.
2. The thermosphere is another name for the ionosphere, as there is an increase in temperature. The ionosphere is the outermost part of the earth’s atmosphere.
3. Many charged particles or ions are present in this sphere of the earth’s atmosphere. Ionisation occurs due to the absorption of the UV rays and other high-energy radiation coming from the sun.
4. Electromagnetic waves bend to diverge towards the earth and thus help in skywave propagation.
5. In this mode of skywave propagation, the frequency ranges from 1710kHz – 40MHz.

Important Quantities of Waves:

Wave speed (c): 

The speed of the wave in a vacuum is 3 x 108 m/s. Wave speed depends on the wave’s frequency, relative permeability, and relative permittivity of the matter. In other words, wave speed depends on the medium.

Wavelength (λ):  

It is the distance between the 2 consecutive crests or troughs in a wave.

Frequency (f): 

The frequency is the inverse of time t for 1 complete cycle of oscillation of a wave.

The relation between frequency and wavelength is as follows:

C = f ƛ

Amplitude (a): 

It is the maximum displacement of the particles of the medium from their equilibrium position. It may be negative or positive.

Reflection: 

The angle of incidence is equal to the angle of reflection for the electromagnetic waves. Change in the direction of the electromagnetic wave is such that it returns to the same place from where it started.

Absorption: 

The energy of the electromagnetic wave changes from one form to another on interaction with the matter. E.g., electric to heat energy.

Transmission: 

Electromagnetic waves need a medium for their propagation.

Intensity: 

It is the mean power travelling through an area perpendicular to the wave direction.

Polarisation: 

The electric part of the wave having only one swinging plane is called linear polarised.

Transducer: 

A device that converts energy from one form to another is called a transducer. E.g., an electrical transducer converts physical variables like pressure, displacement, force, and temperature into electrical signals.

Signal types: 

Transmission of the information in electrical form is called signal. The signal is of 2 types: 

1. Analogue signal:

• Variable voltage and current
• The sine wave is a fundamental analogue signal.
• Example: Sound and picture signals in T.V.

1. Digital signal:

• Based on step value
• 0 represents the binary level, and 1 represents the highest level.
• They use universal coding methods like BCD and ASCII.

Speed (v) of a travelling wave equation (wave equation):

v = υλ = λ / T

Properties of Electromagnetic Waves:

1. Electromagnetic waves travel at a speed of light, ｃ= 3 x 108 m/s.
2. These waves do not require a medium for their propagation.
3. Electromagnetic waves show the properties of interference and diffraction. One can easily polarise them.
4. Electric and magnetic fields cannot easily deflect these waves.
5. Electromagnetic waves travel in transverse form.

Electromagnetic Radiation

Electromagnetic waves sometimes propagate without any influence of the moving charges due to the distance between the 2 charges. They are thus associated with electromagnetic radiation. 

Electromagnetic waves carry energy (E), momentum (p), and angular momentum (w).

Electromagnetic Spectrum

The position of the electromagnetic wave in the electromagnetic spectrum is determined either by its frequency or its wavelength. In the electromagnetic spectrum, waves with their increasing frequency and decreasing wavelength are as follows:

Radio waves → microwaves → infrared radiation → visible light → ultraviolet radiation → X-rays → gamma rays

Conclusion

Electromagnetic waves are ubiquitous and are used presently in modern technologies: AM and FM radio, wireless networks, ovens, automatic door openers, etc. Besides these technologies, many other devices use electromagnetic waves to transmit their data and signals. 

Electromagnetic waves carry energy and momentum that show their duality nature, i.e., wave and particle-like properties on frequency and wavelength. All 7 electromagnetic waves are arranged in an electromagnetic spectrum. Electromagnetic waves travel at a speed of light, c = 3 x 108 m/s.

Electromagnetic waves propagation takes place mainly via 3 modes of propagation: ground, sky, and space. Satellite communication and television broadcasts transmit their signal through sky mode of propagation.