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Electromagnetic Waves

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Physics Part I

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Class 10 Class 12

Write any four characteristics of electromagnetic waves. Give two uses of (i) radio-waves (ii) Microwaves.

Characteristics of electromagnetic waves: 

(i) Electromagnetic waves are produced by accelerating or oscillating charge.
(ii) E.M. waves do not require any material medium for their propagation.
(iii) E.M. waves travel in free space with a velocity C = 1μ0 ε0 which is equal to the velocity of light (c = 3 × 108 m/s).
(iv) E.M. waves are transverse in nature.

Uses of Radio waves:

(i) They are used in radio and TV communication systems.
(ii) Cellular phones use radio waves to transmit voice communication in the ultrahigh frequency (UHF) band.

Uses of Microwaves:

(i) Microwaves are used in Radar systems for aircraft navigation.
(ii) Microwave ovens are used for cooking purposes.



The oscillating magnetic field in a plane electromagnetic wave is given by
By = (8 × 10-6) sin [2 × 1011 t + 300 πx] T
(i) Calculate the wavelength of the electromagnetic wave.
(ii) Write down the expression for the oscillating electric field.  

Given equation for oscillating magnetic field is,
By = (8 × 10-6) sin [2 × 1011 t + 300 πx]T . 

Comparing the given equation with the equation of magnetic field varying sinusoidally with x and t
By = B0 sin2π xλ+2π tT 

we get,
                 2πλ = 300 π 

                  λ = 2300 = 0.0067 m 

and               B0 = 8 × 10-6T 

(i) Wavelength of the electromagnetic wave,  λ = 0.0067 m

(ii) Magnitude of electric field is calculated by,

          E0 = CB0       = 3 × 108 × 8 × 10-6        

              = 24 × 102 = 2400 Vm-1
 The required 
expression for the oscillating electric field is,

       Ez = E0 sin 2π xλ+2π tT     = 2400 sin (300π x + 2 × 1011 t) V/m.


Distinguish between sky wave and space wave propagation. Give a brief description with the help of suitable diagrams indicating how these waves are propagated. 

Sky waves. A radio wave transmitted towards the sky and reflected by the ionosphere towards the desired location of the earth is called a sky wave.
Space waves. A radio wave that travels directly from a high transmitting antenna to the receiving station is called space wave.
Diagram. Various propagation modes of electromagnetic waves (EMW) is shown below.

Sky waves. A radio wave transmitted towards the sky and reflected by


The velocity of light in vacuum can be changed by changing
  • frequency
  • amplitude
  • wavelength
  • None of these


None of these

The frequencies of X-rays, Y-rays and ultra violet rays are respectively a, b and c. Then
  • a < b, b > c
  • a > b, b > c
  • a > b, b < c
  • a < b, b < c


a < b, b > c