Electromagnetic Waves

Electromagnetic waves are the form of visible light that enables us to view the world around us. There are various types of electromagnetic waves out of which you are infrared rays, microwaves, gamma rays, ultraviolet rays, etc. Every wave has a distinctive characteristic, Range of distinctive frequency and wavelength and hence is used in different realms. 

Maxwell was the first scientist to predict the presence of electromagnetic waves. These waves were produced and detected by Hertz. Another scientist called JC Bose produced an electromagnetic range of wavelength between from 5 to 25 milimeter. Successful transmission of electromagnetic waves up to few kilometres was done by Macaroni.

Source of Production

An Electromagnetic wave is a unique wave with three components namely- the magnetic field B, the electric field E the direction of wave motion, x. 

The electromagnetic waves are produced by accelerated charged particles and composed of electric field and magnetic field vibrating transversely and sinusoidally perpendicular to each other. Such waves are produced in the following physical phenomena-

• Accelerating charge produces both electric field and magnetic field which vary with space and time and hence forms electromagnetic wave
•  In LC oscillation and accelerating charge emits electromagnetic wave of same frequency 
•  Electromagnetic wave, gamma-rays is are produced during de-excitation of nucleus in radioactivity

Electromagnetic waves are produced by accelerated charge and oscillating charge. These waves consist of electric field component, E and magnetic field component,B both are perpendicular to each other as well as to the direction of propagation. The phase difference between the two fields is zero. 

Direction of propagation of electromagnetic waves can be represented as-

 E x B (Cross Product of electric field and magnetic field). 

The amplitude of electric field and magnetic field are related to each other as-

B0 = cEo  (c = speed of light)

Characteristics

 The electromagnetic waves carry very unique characters, as listed below-

• These waves do not require any material medium for propagation,This means that electromagnetic waves can travel without any medium and like longitudinal waves and transverse waves. For Example sound waves require air as a medium to travel.
•  The velocity of an electromagnetic wave in a medium is decided by electric and magnetic properties of the medium and not by the amplitude of the electric and magnetic field vector. The speed of electromagnetic wave in a medium can be calculated by- 

c = 1 / √μo εo

μo is permittivity of free space,

∴ c = Eo / B0

where, Eo and Bo are maximum values of electric and magnetic field vectors

• In free space these waves travel with the speed equal to light, c = 3×108 m/s

As in vacuum or in free space there is no influence on the electric field vector and the magnetic field vector of the electromagnetic wave.

• The energy carried by electromagnetic waves is equally divided between electric field and magnetic field and the total average energy density can be calculated as-

UE = (1/2) εo E2 = (1/4) εo E2o

where Eo and Bo are maximum values of electric and magnetic field vectors

• The electric field vector of an Electromagnetic wave produces optical effect and hence is known as the light factor
• The electromagnetic wave is not deflected by electric fields as well as magnetic fields because it consists of uncharged particles called photons

 The average intensity can be calculated by – 

I = (1/2) εo E2oc

• Electromagnetic waves carry energy as well as momentum- 

p= U/c,

p= momentum

U= energy carried by electromagnetic wave in free space

c= speed of electromagnetic wave in free space

• Radiation pressure- radiation pressure is defined as the force exerted by electromagnetic waves on a unit surface area
• Two physicist Nicols and Hull, measured radiation pressure of visible light, they found it to be 7×10-6 N/m2

The radiation pressure can be measured in two ways-

• The surface absorbs the radiation falling on it then the radiation pressure P, is given by-

 P= I/c

• If the surface completely reflects the radiation falling on it then the radiation pressure, P is given by-

P= 2I/ 

Types of Electromagnetic Waves

Orderly distribution of electromagnetic radiations according to their frequency or wavelength is called electromagnetic spectrum. There exist no clear-cut division between one kind of wave and the other and the classification is solely based on how these waves are produced or detected.

We have already discussed that electromagnetic waves experimentally were discovered by Hertz. By the end of the 19th century, visible light, ultraviolet waves, infrared waves, X-rays, and gamma rays had also been discovered. These waves are distinctively used for various purposes. 

For instance, radio waves are extensively used in radio and TV communication, microwaves are used in microwave ovens, infrared waves are those which are produced by hot bodies and thus, are used to treat muscular strain, to provide electrical energy to satellites and the like. Similarly, ultraviolet rays are used to kill germs as well as sterilize surgical instruments, but they are extremely harmful for humans and aquatic animals if produced in large quantities. 

The Ozone layer in the stratosphere layer of the atmosphere above Earth is majorly responsible for keeping away the harmful ultraviolet radiation from the Sun.

Since our childhood we have been aware that X-rays, extraction of electromagnetic radiation, is used for detecting cracks and fractures in medicine. The other set of electromagnetic waves used for treating cancer therapy, Famously known as radiotherapy, come under the category of gamma rays. 

Not only this, these waves have quite a significant place in our day to day life, and have been a major part of the scientific development we have today. 

Conclusion 

The basic difference between various types of electromagnetic waves lies in their wavelengths of frequencies since all of them travel through vacuum with the same speed. Consequently the Waves differ considerably in their mode of interaction with the matter. 

Infrared waves with frequencies lower than that of visible light vibrate not only the electrons but the entire molecule or atom of a substance, this vibration increases the internal energy and hence the temperature of that particular substance this is why, why these waves are called heat waves. The centre of sensitivity to our eyes coincides with the centre of the wavelength distribution of the sun, it is because humans have evolved with visions most sensitive to the strongest wavelength from the Sun. 

These electromagnetic waves are responsible for many scientific developments which we see today and have been a major part of our lives, as they have many practical implications.