Saturday, 14 July 2012



The oscillating electric & magnetic field which regenerate each other & propagate through space with speed of light are known as electromagnetic waves.

Properties of electromagnetic waves

1.      E.M waves require no material, medium for propagation .
2.      E.M waves are transverse in nature .
3.      Speed of E.M wave is different in different media & is given by : ν=1/√μϵ . It is maximum in air/vacuum & equal to
     c = 1/√μ0ϵ0 =3 x 108 m/s
4.      Ratio of electric field to magnetic field gives speed .


The orderly distribution of different type of E.M. waves according to their specific characteristic like frequency wavelength etc. is called electromagnetic spectrum .


1.      Radio waves
2.      Micro waves
3.      Infrared waves
4.      Visible rays
5.      Ultraviolet rays
6.      X rays
7.      Gama rays.
 In ascending order of frequency for different type of waves :                                                        
Radio waves <Micro waves < Infrared waves < visible rays< ultraviolet rays < X rays < Gama rays .


Types of E.M. waves

Wavelength range
Frequency range


Way of detection
Applications/ effects

Radio waves

Greater than 10cm
500kHz to 1000mHz
Rapid acceleration & deacceleration of electrons
Reciver aerial
Radio & TV communication

Micro waves

1mm to10 cm
109to 1012Hz
Klystron valve Magnetron valve
Point contact diodes
Used in radar system
Micro wave oven
Infrared waves
700nm to 1mm
1012 to 4x1014Hz
Vibration of atoms & moleculs
Infraphotographic film
Cause heating effect
Physical therapy
Remote switches 
Visible rays
400nmto 700nm
4x1014 to7x10 14Hz
Electrons in atoms emit light when they move from one energy level to lower energy level
The eye, photo cell &
Photographic films
To see objects
To convert in electric energy
To take photographs optical signal
Ultraviolet waves
6x10-10m to 400nm
Inner shell electron in atom moving from one energy level to lower energy level
Photocell Photographic films
LASIK eye surgery &
To kill germs

X rays
10-3nm to1nm
3x1016to 3x1020Hz
X ray tube or innershell electron
Photographic film Geiger tubes Ionisation chamber
Diagnostic measure,
Treatment of cancer &
To locate the fracture of bones
Gama rays
Less than 10-4to10-10m
Greater than 3x1020or1019Hz
Radioactive decay of nucleus
Photographic film Geiger tubes Ionisation chamber
To destroy cancer cells
Experimental study

  “  Electromagnetic wave”2
1-State any four properties of electromagnetic waves ?
2-Arrange following  radiation is the descending order of the wavelength  ?
   Gama rays , infrared rays microwaves , yellow light , radio waves .                                                                             
 3-write an expression for speed of electromagnetic waves in free space .                                 
 4-give one use of following .-
a.      infrared rays
b.      gamma rays
c.      microwaves
d.      ultraviolet radiation .
 5-Deduce an expression for the distance which the T.V. signal can directly be received from a T.V. tower of height .
6- By which way the X- rays and  gamma rays  can be produce and detected write one use of  each .                     
7- What oscillate in E.M. Waves? Give four example of E.M. waves .
8.- identify the part of the electromagnetic spectrum which is
a.      Suitable for radar system used in air craft navigation
b.      Which is adjacent to low frequency end of the E.M.       spectrum .
c.      Produced in nuclear reaction
d.      produced by bombarding a metal target by high speed electron
  9- Name the radiation of the electromagnetic spectrum which are used in
1.      War far to look through fog.
2.      Radar of geostationary satellite
3.      studying the structure and properties of atom and molecules
4.      Killing germs.
5.      To take photograph of internal part of human body.
6.      To produce intense heating effect.
    10 – why sky wave are not used in transmission of T.V. signal .Why the transmission of the signal using sky wave is restricted to up to 30 MHz .

Dual nature of radiation and matter

                               Photo electric emission- 

when light of suitable frequency is incident on a metal surface, electrons are emitted from the metal surface. The emitted electrons are called photoelectrons and this phenomenon is called photo electric emission. 
                               Einstein’s photo electric equation: 
Let a photon of frequency ν having energy hν is incident on the metal surface. The energy of photon divided into two parts-
  1. One part of this energy is absorbed exceed the minimum energy needed for the electron escape from the metal. This energy is called work function of metal and is denoted by ωand frequency of this energy is called threshold frequency and ωo= hνo
  2. The second part of photon energy is used to accelerate the free electron i.e. in the form of kinetic energy.
Therefore-    hν = ωo + K.E.

                             hν = hν+ K.E.

                  K.E. = hν  -  hνo

This is called Einstein’s photo electric equation.                                                                                                     

                                    Particle nature of light:  
Electromagnetic radiation follows particle nature as photon as below-
  • An interaction of radiation with matter, radiation behaves as if it is made up of particle called photons
  • Each photon has energy E = hν, momentum = hν/c and speed c (speed of light )
  • All photons of light of a particular frequency ν or wavelength λ, have the same energy
                                            E = hν = hc/λ
                  and momentum
                                              p = hν/c =h/λ
  • Photon energy is independent of intensity of radiation
  • Photons are electrically neutral and not deflected by electric and magnetic field.
  • In a photon particle collision the total energy and total momentum are conserved but number of photon may not be conserved.

                                                                 Wave nature of matter-
     According to de-Broglie, Moving particle of matter should display wave like properties under suitable
      conditions. The wavelength λ associated with a particle momentum p is given as-
                                                              λ = h/p = h/mv    
      Where m   is the mass of the particle and v is speed of particle. ν  is called de-Broglie wavelength.

                                               de-Broglie wave length of electron-  

Consider an electron of mass m, charge e accelerated from rest through a potential V. The kinetic energy K of the electron equals the work done (eV) on it by the electric field:  
                                                                     K =eV
                                                Now             K = 1/2mv2 = p2/2m
P =√2mK
   = √2m eV 
Therefore the de-Broglie wavelength λ of the electron is given by
λ = h/p = h/√2mK = h/√2m eV
Putting the value of h, m, and e we get
λ = 1.227/√v   nm     where v is the magnitude of accelerating potential in volts.

                                 Uncertainty principle:-
It is not possible to determine the position and momentum of any particle at the same time exactly. If there is uncertainty in position is Δx and in momentum Δp then
                                          Δx. Δp ≈h/2Π

                       Davisson and Germer Experiment

The wave nature of electron was first experimentally verified by C.J. Davidson and     L.H. Germer in 1927 and independently by J.J.Thomson in 1928, who observed diffraction effects which beam of electrons scattered by crystals.
Experimental arrangement as below-

Here electron beam emitted by the filament F with the help of L.T. Battery and this beam is accelerated with the help of H.T. Battery. The electron beam fall on the nickel crystal. The electron are scattered in all directions by the atom of the crystal.
The intensity of the electron beam, scattered in a given direction is measured by the electron detector. The detector can be moved on a circular scale and is connected to a sensitive galvanometer which records the current. The intensity of the scattered electron beam is measured for different values of angle of scattering and which is the angle b/w the incident and the scattering electric beams. The variation of intensity (I) of the scattered electron with the angle of scattering and is obtained for different accelerating voltages.
The experiment was performed by varying the accelerating voltage from 44V to 68V .
It is observed that maximum value of (I) of the scattered electron for an accelerating voltage of 54V at a scattering angle θ = 500
The wavelength of matter wave is associated with electron in experimentally obtained   λ     = 0.165 nm ------------------ (1)
The de-Brogli wavelength λ associated with electron using λ = h/p =1.227/v nm.   For V=54V
       λ = 1.227/54 nm.
        λ = 0.167 nm.--------------------  (2)

       From experimental result and calculated result we see that both are approximately same. This shows that matter wave associated with moving electron.

Important Questions:-

  1. Define work function function for a given metallic surface?
  2. Two metals m1 & m2 have work functions 2 eV and 4 eV respectively. Which of the two has a higher threshold wavelength for photo electric emission?
  3. A photon has velocity ‘C’ and frequency v. write down the expression for its wavelength?
  4. On what factor does the energy carried by a photon of light depend?
  5. Name the experiment which establishes the wave nature of a particle?
  6. A photon and an electron have the same wavelength. Which particle is moving faster?
  7. What is the de-Broglie wavelength of a 3 Kg object moving with a speed of 2 m/s?
  8. Explain laws of photoelectric emission on the basis of Einstein’s photo electric equation?
  9. What is photoelectric effect? Calculate the threshold frequency of photon for photoelectric emission from a metal of work function 0.1 eV?
  10. what is the-
·         momentum
·         speed and
·         de-Broglie wavelength of an electron with kinetic energy of 120 eV?

  1. Describe deviation and Germer experiment to establish the wave nature of electrons. Draw a labeled diagram of the apparatus used.
  2. a.  Define terms-
·         Work function
·         Threshold frequency
·         Stopping potential with reference to photoelectric effect.

              b.    Calculate the maximum kinetic energy of electrons emitted from a photo sensitive surface a work function 3.2 eV for the incident radiation of wave length 300 nm.