ELECTROMAGNETIC WAVE THEORY ,class-11

Electromagnetic Wave Theory[Wave nature of electromagnetic radiations]

This theory was put forward by James Clark Maxwell in 1864.
MAIN POINTS

1. The energy is emitted from any source (like the heated rod or the filament of a bulb through which electric current is passed) continuously in the form of radiations and is called the radiant energy.
2. The radiations consist of electric and magnetic fields oscillating perpendicular to each other and both perpendicular to the direction of propagation of the radiation.
   
   
   
   
3. The radiations possess wave character and travel with the velocity of light (i.e.,nearly 3 ×10⁸ m/s).That is why the radiations are called Electromagnetic radiations or Electromagnetic waves.
4. These waves do not require any material medium for propagation.

Some important characteristics of a wave

Wavelength(λ ), frequency (ν) and velocity (c)are the main characteristics of a wave.

• WAVELENGTH :- It is defined as the distance between any two consecutive crests or troughs.
  It is represented by λ (lambda) and is expressed in angstrom (Å) or m or cm or nm (nanometre) or pm (picometre).
  1Å = 10⁻⁸cm = 10⁻¹⁰m, 1 nm = 10⁻⁹m , 1 pm = 10⁻¹²m
• FREQUENCY :-It is defined as the number of waves passing through a point in one second.
  It is represented by v (nue)and is expressed in Hz (hertz) or cycles/second or simply sec⁻¹ or s⁻¹.
  1Hz = 1 cycle/sec
• VELOCITY :-It is defined as the linear distance travelled by the wave in one second.
  It is represented by c and is expressed in cm/s or m/s.
  Two more charecteristics of a wave :-
  
• AMPLITUDE :-It is the height of the crest or the depth of the trough. It is represented by 'a' and is expressed in the units of length.
• WAVE NUMBER :- It is defined as the number of waves present in 1cm length.
  It is equal to the reciprocal of the wavelength.
  It is represented by    
  Its unit is cm⁻¹ or m⁻¹.
• TIME PERIOD :-It is the time taken for one wave to pass through a point. It is the reciprocal of frequency.
  
  
  
  
  
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Relationship between velocity, wavelength and frequency of a wave

velocity = frequency × wavelength c = v × λ

ELECTROMAGNETIC SPECTRUM

When electromagnetic radiations are arranged in order of their increasing wavelengths or decreasing frequencies, the complete spectrum obtained is called electromagnetic spectrum.

Questions :-

1. The Vividh Bharti station of all India Radio,Delhi, broadcasts on a frequency of 1,368 kHz. Calculate the wavelength of the electromagnetic radiation emitted by transmitter. Which part of the electromagnetic spectrum does it belong to ?
2. The wavelength range of the visible spectrum extends from violet (400nm) to red (750nm). Express these wavelengths in frequencies (Hz). ( 1nm = 10⁻⁹m)
3. Calculate (a) wavenumber and (b) frequency of yellow radiation having wavelength 5800 Å.
4. Calculate wavelength, frequency and wavenumber of a light wave whose period is 2.0 ×10⁻¹⁰ s.