Electromagnetic Waves | Part 1 | Definition, Characteristics, Formula, etc...

Electromagnetic Waves 

   Is defined as, Time varying electric and magnetic fields, propagating in space.

E.g. Infrared rays, microwaves, radio waves, X-rays, UV rays etc.

They are mostly produced by accelerated electric charge. 

  Hertz's Experiment 

For production of electromagnetic waves

   1. An experimental setup used by Hertz's to produce and detect electromagnetic waves.

         

   2. The transmitter consists of two spheres S1 and S2 located near the end of two straight rod A and B seprated by a spark gap S.

   3. With the two rod connected to an induction coil I, spark jump across the gap S, giving rise to an oscillatory current in A & B. The sphere S1, S2 act as plates of a capacitor and the rod A and B provide inductance. Hence, the transmitter act as an oscillator  circuit.

   4. The recevier or detector, consists of a single loop of wire with tiny spark gap at R. 

   5. This circuit too is an oscillating circuit with the spark gap acting  as a capacitor and the loop providing the inductance.

   6. Tuning the frequency so that of the receiver is done by sliding the sphere S1, S2, along the rod A and B. When the two circuits are tuned, a spark appear across R, whenever a spark passes across S.

   7. Wavelengths of electromagnetic waves were found to be 6m.

   8. These waves under go reflection, refraction, interference etc. Similar to light.

Characteristics Of Electromagnetic Waves

   1.Electric field vector and magnetic field vector vibrate perpendicular to each other and also to the director of propagation of wave 

i.e., Electromagnetic Waves (E.M.) are transverse in nature

   2. E.M. waves propagate in the form of time varying electric and magnetic fields.

   3. The ratio of the amplitudes of electric and magnetic fields is always constant and it is equal to velocity of the electromagnetic waves
                                                              c =  E 

                                                                     B

   4. The energy of E.M. waves is equally distributed among the electric and magnetic field vector. 

   5. E.M. waves are produced by accelerated electric charges. 

   6. E.M. waves do not require any material medium for their propagation. They can travel through vacuum as well as through solids, liquids, and gases.

   7. The relation between velocity (c), frequency(ν) and wavelength (λ) of the E.M. waves in vacuum is given by c = νλ

   8. E.M. waves travel with the speed of light in vacuum and their velocity (c) is given by, 

 c =    1         =                        1                        

       √μoεo             4π × 10^-7 × 8.85 ×10^-12

                                             c = 3 × 10^8 m/s

 Where, and are the permittivity and permeability of free space. 

    9. In a given material medium, the velocity (vm) of E.M. waves is given by, 

       vm =   1  

              √με

Where  μ  = Permeability of the given medium

             ε  = Permittivity of the given medium

   10. E.M. waves obey the principle of superposition of waves.

   11. E.M. waves exert pressure on the surface upon which they are incident.

   12. E.M. waves obey  the laws of reflection and refraction.

   13. The low frequency E.M. waves are unaffected by the external electric and magnetic fields.

 Transverse nature of E.M. waves

    1. The electric and magnetic fields are mutually perpendicular to each other. If the accelerated charge is oscillating, both the electric and magnetic fields vary with time and they travel outwards from the charge in the form of E.M. waves.

     2. If E is along the Y-axis and B is along the Z-axis, the direction of propagation is along E × B i.e., along the X-axis 

             

     3. A single E.M. wave propagating perpendicular to both electric and magnetic fields in magnified form as shown below

             

     4. The electric field and magnetic field vary sinusoidally with x and is given by,

                                            Ey = Eo sin(kx - ωt)

                                            Bz = Bo sin(kx - ωt)

Where, Eo = amplitude of electric intensity E

            Bo = amplitude of magnetic induction B

             k = 2π  = Propagation constant 

                     λ

             λ = wavelength of oscillation

     5. Both electric and magnetic fields attain their maximum and minimum values at the same time and at the same point in space 

      6. From the figures it is observed that propagation of electromagnetic fields is in the direction of E × B. 

As the electric and magnetic fields are mutually perpendicular to each other and to the direction of wave propagation, the electromagnetic waves are transverse in nature.

 Quick Question. #QQ

Q.Is it possible to polarise the E.M. waves?

Ans. Yes, E.M. waves can be polarised. A beam of electromagnetic waves is unpolarised, but if the vibration of the electric field vector are confined to one plane containing the direction of propagation, then the waves are said to be plane polarised or linearly polarised.

To view Electromagnetic Waves  Part 2 click the link below
Electromagnetic Waves | Part 2 |