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Sunday, September 24, 2017

CBSE PHYSICS - PROPERTIES AND USES OF ELECTROMAGNETIC WAVES

What are electromagnetic waves?

An electromagnetic wave is a wave radiated by an accelerated charge  and which propagates in space through electric and magnetic fields, oscillating perpendicular to each other and to the direction of propagation of the wave.

Electromagnetic Waves

Characteristics of electromagnetic waves:

(i) The electromagnetic waves travel in space with the speed of light.
(ii) Electromagnetic waves or neutral, so they are not deflected by electric and magnetic fields.
(iii)The ratio of magnitudes of electric and magnetic field vectors in free space is constant equal to c,
E B = 1 μ o ε o =c=3X 10 8 m/s 
(iv)The directions of oscillations of A and B fields are perpendicular to each other as well as perpendicular to the direction of propagation of wave so the electromagnetic waves are transverse in nature.

What is electromagnetic spectrum?

The orderly distribution of electromagnetic waves in accordance with their wavelength or frequency into groups having different properties is called electromagnetic spectrum.  
Different parts of Electromagnetic spectrum

Different parts of the electromagnetic spectrum:

  1. Radio waves - These are the e.m. waves of longest wavelength and minimum frequency.
Wavelength range 600 m to 0.1 m
Frequency range 500 kHz to 1000 MHz
Source Accelerated motion of charge in conducting wires or oscillating
                                       circuits  
                                                                      
Discovered by Marconi in 1895
Properties Reflection, diffraction
Uses of radio waves:
  • In radio and television communication system
  • In radioastronomy.
   2.   Microwaves -  They are the electromagnetic waves having wavelength next smaller to radio
waves.
Wavelength range 0.3 m to 10-3m
Frequency range 109 Hz to 1012 Hz
Source Oscillating currents in special vacuum tubes like magnetrons and
Gunn diode.
Properties Reflection, refraction, diffraction and polarization, due to the
shorter wavelength they can travel as a beam in a  signal.
Uses of microwaves:
  • In radar system for aircraft navigation
  • In long distance communication system by a geostationary satellites
  • In microwave ovens.


3. Infrared waves -  These radiations like close to the low frequency or long wavelength of the
visible spectrum. infrared waves produced heating effect so they are also known as heat waves
or thermal radiation.
Wavelength range 5 X 10-3 m to 10-6 m
Frequency range 1011 Hz to 5 X 1014 Hz
Source Hot bodies and molecules
Discovered by William herschel in 1800
Properties Heating effect, reflection, refraction, diffraction
Uses of infrared waves:
  • In the remote control of a TV
  • In greenhouses to keep the plants  warm
  • To take photographs in fog
  • In reading secret writings on the ancient walls
  • To study the molecular structure


4. Visible light - It is a very small part of EM spectrum towards which human eyes  are sensitive.
Wavelength range 8 X 10-7 m to 4 X 10-7 m
Frequency range 4 X 1014 Hz to 7 X 1014 Hz
Source Radiated by excited atoms in ionized gas
Properties Reflection, refraction, interference, diffraction,polarization, Photoelectric effect.
Uses of visible light:
  • It provides us the sensation of light so ,we are able to see the things
  • It can cause chemical reactions.

5.  Ultraviolet light -  This  region  of EM spectrum has wavelength just shorter than visible light
and it can be detected just beyond the violet end  of solar spectrum.
Wavelength range 3.5 X 10-7 m to 1.5 X 10-7 m
Frequency range 1016 Hz to 1017 Hz
Source High Voltage gas discharge tubes, the sun
Discovered by Ritter  in  1800
Properties Effect on photographic plate, ionization, highly energetic,
tanning of human skin.
Uses of ultraviolet light:
  • In food preservation.
  • In the study of invisible writings and fingerprints.
  • In the study of molecular structure.

6. X - rays -  These  EM waves have wavelength just shorter than ultraviolet light.
Wavelength range 100 Å to 0.1 Å
Frequency range 1018 Hz to 1020 Hz
Source Sudden deceleration of fast moving electrons by a metal target
Discovered by Rontgen In 1895
Properties Effect on photographic plate, ionization of gases, Photoelectric
effect, more energetic than UV rays.
Uses of X - rays:
  • In medical diagnosis  because X rays can pass through flash but not through bones.
  • In the study of crystal structure.
  • For detecting faults ,cracks, flaws in the metal products.
  • To detect explosives, diamond, Gold in the possession of smugglers.
  • In radiotherapy to cure skin diseases.

7. Gamma rays -  These are EM radiations of highest frequency range and lowest wavelength
range. these are most penetrating EM waves.
Wavelength range 10 -14 m to 10-10 m
Frequency range 1018 Hz to 1 022 Hz
Source Radioactive nuclei and nuclear reactions. Co - 60 is a pure
γ - ray source.
Discovered by Henry becquerel in 1896
Properties Effect on photographic plate, fluorescence, ionization,
diffraction, high penetrating power.
Uses of gamma rays:
  • In radiotherapy for the treatment of tumors.
  • To initiate some nuclear reactions.
  • To preserve food stuffs for a long time because gamma rays can kill microorganisms.
  • To study structure of atomic nuclei


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