Propagation of EM Waves

Introduction

To comprehend the propagation of electromagnetic waves, it is necessary to first define what an electromagnetic wave is. These waves are produced as a result of vibrations between an electric and magnetic field. The speed of electromagnetic wave propagation through the vacuum is 3*10⁸ ms^-1, and the speed of electromagnetic wave propagation through any medium is less than that of the speed in the vacuum. Electromagnetic Waves, as well known as Electromagnetic Radiations, are described as superimposed oscillations of an Electric and a Magnetic Field in space with propagation directions perpendicular to both. In basic terms, electromagnetic waves are oscillations caused by the intersection of an electric and a magnetic field. The propagation direction of such waves is perpendicular to the force direction of either of these fields. These, like all waveforms, have some properties.

Overview

The absorption and reemission of wave energy by the material’s atoms is the mechanism of energy transport through a medium. The energy of an electromagnetic wave is absorbed when it strikes the atoms of a material. The electrons within the atoms vibrate as a result of energy absorption. After a brief period of vibrational motion, the vibrating electrons generate a new electromagnetic wave of the same frequency as the first. Whereas these vibrations last only a few seconds, they cause the wave’s motion through the medium to be delayed. When an atom re-emits the energy of an electromagnetic wave, it travels through a small region of space between atoms. The electromagnetic wave is absorbed, transformed into electron vibrations, and then reemitted as an electromagnetic wave when it reaches the next atom.

The optical density of a material medium determines the actual speed of an electromagnetic wave through it. Because of the absorption and reemission process, different materials cause varying amounts of delay. Furthermore, different materials have their atoms more closely packed, resulting in less distance between atoms. Those certain two factors are affected by the material through which the electromagnetic wave is travelling. As a consequence, the speed of an electromagnetic wave is affected by the material through which it travels.

Propagation of EM Waves

Properties of Electromagnetic Wave Propagation

• Such waves have the same speed as light.
• All such waves do not need a medium to propagate.
• Electromagnetic waves move in a transverse direction.
• Electric and magnetic fields have no effect on electromagnetic waves.
• Those certain waves have the ability to be polarised.
• Interference and diffraction occur in electromagnetic waves.

The wavelength (λ) and frequency (v) of electromagnetic waves can be related as follows:

c = v.λ,

for which c is the wave’s velocity

Let us look at the production of X-rays as an example of electromagnetic wave propagation.

X-rays were indeed electromagnetic waves with wavelengths ranging from 0.001 to 10 nanometers (1 nanometer = 10^-9 metres). They have a wide range of applications, from medical purposes, such as detecting an anomaly inside the body, to security at airports or public places.

X-rays have been produced within an Xray tube, which is nothing more than a tube with a vacuum and a high potential difference at its ends. It thus creates a stream of high-velocity electrons to strike a target anode, which is typically made of tungsten or another metal depending on the wavelength required.

When electrons collide with a target, the high energy they possess is transmitted to the anode in the form of photons, resulting in the generation of x-rays. Then let us look at some EM wave propagation methods: The different layers are the mesosphere, troposphere, and ionosphere.

Such layers are used for EM wave propagation, and EM waves can travel in any of the three ways discussed below:

• Ground Wave:

Used only for low-frequency transmission, typically less than 1 MHz. Such a form of propagation employs the use of large antennas, the order of which is equivalent to the wavelength of the waves, and it propagates through the ground or troposphere. This method is not used to send signals over long distances. This leads to severe attenuation, which increases as the frequency of the waves increases.

• SkyWave:

Used in the propagation of electromagnetic waves with frequencies ranging from 3 to 30 MHz. It makes use of the ionosphere, which is so named due to the presence of charged ions in a region 60 to 300 km from the earth’s surface. Within a specific frequency range, these ions act as a reflecting medium for radio or communication waves. We have used this ionosphere property for long-distance wave transmission with minimal attenuation and signal strength loss.

Some other important consideration is the angle at which these waves are emitted from the ground. A transmitter radiates the EM Waves at a critical angle to ensure total reflection to the ground, similar to how optic waves are total internal reflection; otherwise, the waves may escape into space. The skip distance is the distance between the two points where the wave transmission occurs.

• Space Wave:

Line of Sight communication, also known as LoS, is used. This method of propagation is used for space satellite communication and very high-frequency waves. It entails sending a signal from the transmitter to the receiver in a straight line. We must ensure that, over very long distances, the height of the transmission tower is sufficient to prevent waves from colliding with the earth’s curvature, resulting in attenuation and loss of signal strength.

There seems to be an important relationship for determining the height of the antennas and their corresponding transmission distance, which is given by:

Dm=(2RH_t)^-1/2 +(2RH_r)^-1/2

in which Dm denotes the distance between the two antennas; R denotes the earth’s radius, which is 6400 km; H_t denotes the height of the transmission antenna; and H_r denotes the height of the receiver antenna. Another important relationship to consider when determining the range of transmission (Dt) for a given antenna of height H_t is:

D_t=(2RH_t)^-1/2

In turn, the changing magnetic field induces an electric field, resulting in a series of electrical and magnetic oscillations that combine to produce a form that propagates as an electromagnetic wave. The EM propagation in the radar frequency range really does have significant advantages for security applications that require low visibility: radar beams can penetrate fog and clouds and thus contribute significantly to security in aviation, shipping, and many other areas.

FAQs

What is meant by propagation wave?

Wave propagation relates to any of the methods by which waves travel. We can distinguish between longitudinal and transverse waves based on the direction of the oscillation relative to the propagation direction. Electromagnetic waves can propagate both in a vacuum and in a material medium.

Do Electromagnetic Waves carry energy?

Electromagnetic waves have the property of being massless. They do, however, carry energy. Even though these waves have no mass, they do have momentum, and their energy is proportional to their frequency.

What are the examples of Electromagnetic Waves?

Electromagnetic waves can be represented by the following examples: Invisible light, Radio waves, Microwaves, Infrared rays, Ultraviolet rays, X-rays and Y-rays