• facebook
  • twitter
  • whatsapp
  • telegram

12. Dual Nature of Radiation and Matter

(Electron Story)

           Fundamental Particle of Electricity: The discovery of electron plays an important role in the development of 'Modern Physics'. While performing certain experiments in the field of electrolysis (passing electricity through liquids and analysing the constituents of liquids). Michel Faraday (who never had even proper schooling) well known as 'Father of electricity' showed that each ion taking part in electrolysis has a fixed charge. Later, it was left to the Irish Physicist, John Stone Stoney to determine the magnitude of this charge. To this unit of charge he gave the name 'Electron'. While passing electricity through gases (when Faraday passed electricity through liquids, why not through gases?). 'Sir William Crookes' in 1870 discovered 'cathode rays' which consist of streams of negatively charged particles.

             Later, in 1897, Joseph John Thomson (well known as J.J.Thomson) continued his experiments on passage of electricity through gases and confirmed the phenomenon observed by William Crookes. He further established that atom in matter has a structure which consists of these negatively charged particles and named them as 'electrons'. J.J.Thomson, well known as 'Father of electron' has established that electron is a 'particle' for which he was awarded Nobel Prize in Physics for the year 1906. It is very interesting to know that his son G.P.Thomson was also awarded Nobel Prize in Physics for the year in 1937 for showing that the electron is a 'wave'.
            'Discovery of electron' has paved the way for a new branch of physics called 'Atomic Physics'.
Electron Bio - Data
             Sir J.J.Thomson under took a quantitative study of cathode rays. He named the cathode ray particle as 'electron'. He discovered a unique common characteristic of these rays - that whatever be the gas inside the discharge tube the ratio of its charge (e) to its mass (m) i.e. e/m of these particles (electrons) was constant. It was observed that these rays could be deflected by both electric and magnetic fields and direction of these deflections showed that the rays were negatively charged.

 J.J.Thomson's value for  (specific charge) for cathode ray particle (electron) was
1.7589 × 1011 coulomb kg-1.
      The ratio of e/m is independent of the material of the cathode and anode, voltage V applied and the nature of the gas inside the discharge tube used in the experiment by J.J.Thomson which shows that electron is a common constituent of all matter.
 Later the charge 'e' on the electron was measured by R.A. Millikan in 1913 by his 'oil drop experiment' to be equal to 1.6 × 10-19 coulomb.
 Knowing the values of '  ' and 'e' for an electron, the mass of the electron was found to be
9.1 × 10-31 kg (so small!!).
 Electrons are found to be travelling with speeds ranging from about 0.1 to 0.2 times the speed of light (3 × 108 ms-1).

Electron Volt (eV)
           Charged particles are given high energy by accelerating them in a very high electric fields. The energy of the particles is measured in terms of electron - Volts.
          'Electron Volt is the energy acquired by an electron in falling through a potential of one Volt.
          Since 1 Joule = 1 coulomb × 1 volt.
          1 electron volt = electronic charge in 1 coulomb × 1 volt.
          1 electron volt = 1.6 × 10-19 Joule.
Electron emission
Different methods of producing electrons:

i) Discharge tube phenomena.
ii) Thermionic emission: Metals consist 'free electrons' which are responsible for their conductivity. By suitably heating (supplying thermal energy) the metals the free eletrons will come out of them.
iii) Field emission: Strong electric fields of the order of 108 V m-1 are applied to a metal and free electrons can be pulled out of the metal.

iv) Photoelectric emission: Substances when exposed to electro magnetic radiation such as visible, ultraviolet, infrared light, X - rays, γ - rays absorb the radiations and electrons are emitted from the surface of the substance.
               This phenomenon is called 'photoelectric effect' and electrons emitted are called photo - electrons.
Photoelectric effect: Einstein's 'NOBEL'
        The world renowned physicist Albert Einstein (1879 - 1955) was awarded the 'Nobel Prize' in Physics for the year 1921 for his theory of 'photoelectric effect' and not for his much acclaimed 'Theory of Relativity'. Both these theories along with the theory on 'Brownian motion' were published by Einstein in 1905 (miracle year). To commemorate the centanary of these theories which have influenced the modern physics, the united nations declared 2005 as World Year of Physics (WYP). The aim of WYP is to raise the world - wide awareness of Physics.

Discovery - Hertz's observations
        In 1887, when Hertz (whose name was immortalized by giving it for the unit of frequency) was producing the first 'radiowaves', he observed that when light is incident on one of the metal plates placed in an evacuated tube, electrons were liberated from the illuminated plate and their strength was found to depend on the intensity of light.
Ultraviolet light - A sponsor
          In 1888, Hallwachs found that when ultraviolet light was incident on a neutral zinc plate it became positively charged.
          When the same light was incident on a negatively charged zinc plate, it is losing its charge.
          When incident on a positively charged zinc plate, it became more positively charged. Then he concluded that only negatively charged particles can be emitted from the surface of the zinc plate under the action of ultraviolet light.
Experimental study of photoelectric effect
         To study the photoelectric effect an experimental arrangement is shown in the figure below.

         In fig (1), two zinc plates S and P enclosed in a vacuum tube are connected to a battery B and an ammeter A. When a beam of light strikes on the plate S which is connected to the negative terminal of the battery, electrons are emitted by the surface of S and are drawn to the anode P maintained at a positive potential with respect to S. The current due to these electrons is detected and measured by the ammeter A. This current is a measure of the number of electrons collected at the anode P.
          The alkali metals like Sodium, Potassium, Cesium emit photoelectrons even to rays from the visible part of the spectrum, where as Zinc, Cadmium etc, emit photoelectrons only to ultraviolet light.

 Light of short wavelength (greater frequency) has more energy and is thus more effective in producing the photoelectrons. The effect is also found in non - metals, but as they are highly electro negative waves of very short wavelength are required.
 Photoelectric effect is also observed in liquids and gases.
More and more experiments
         In 1889, J.J.Thomson, the father of electron showed that e/m value, of the emitted particles was the same as that of electrons.
 This effect was an instantaneous phenomenon.
 The time between the incidence of radiation and the ejection of electrons is of the order of 10-9 seconds.
 The experiments conducted by Richardson and Compton showed that
i) The rate of emission of electrons i.e., the strength of the photoelectric current, is proportional to the intensity of light.
ii) The maximum kinetic energy of the emitted electrons depends only on the frequency of the light and has nothing to do with the intensity of light. These are called Laws of photoelectric effect.

 For every metal, there is a particular minimum frequency of the incident light, below which there is no photoelectric emission, whatever be the intensity of radiation. This minimum frequency which can cause photoelectric emission is called the 'threshold frequency'.
 Threshold frequency is different for different metals for most metals, it lies in the ultraviolet region, but for alkali metals like Potassium, it is in the visible region.
How can a 'wave' emit a particle?
             Photoelectric effect is an interesting phenomena where light is converted into electricity which became a puzzle for classical physics. The wave theory of light, in which light is considered to be a 'wave' had been established just a hundred years before and for a full century had been winning victory after victory. Then, how can a 'light wave' when incident on a metal plate emit a 'particle electron'? The physical theories of the nineteenth century could not account for this.
Einstein enters on the scene!
           In 1905, Albert Einstein a brilliant young man of 26 years old, employed as a clerk in a patent office at Berne in Germany with heroic mind explained the phenomenon of photoelectric effect. This explanation made the down fall of many theories of classical physics. Here Einstein made use of 'Quantum theory of Radiation' propounded by Max Planck in 1900, which was then only five years old but very much neglected.

            Einstein assumed that light interacts with matter in the form of 'Quanta', the discrete packets (particles) of energy called 'Photons' (from the Greek for 'light'). When light incident upon the surface of a metal, the electrons bound to the surface absorb the energy of one Quantum at a time. When a photon is incident on a metal surface, an electron is emitted and the phenomenon is called 'Photoelectric effect' and the electron is called 'Photoelectron'.
Einstein's Photoelectric equation
           Metals consist of 'free electrons' which move about at random inside them. Though the electrons are free, they cannot escape from the metals of their own accord. It requires a minimum amount of energy for a free electron to come out from a metal. This minimum amount of energy is different for different metals and is called 'work function' for the given metal. Usually, the free electrons in a metal do not possess sufficient energy to escape from it.


According to Einstein, when light of frequency υ is incident on a metallic surface, Photons of energy hυ strike the metal (h is Planck's constant). A photon gives its energy hυ to the electron and if this energy exceeds the 'work function' of the metal, the electron is ejected out of the metal. If the energy hυ of the striking photon is less than the work - function, no electron is ejected. The minimum energy required hυ0 where υ0 is the 'Threshold frequency' and it depends on the nature of the metal.
             If the frequency of the incident light is just equal to the threshold frequency i.e., υ = υ0. Then the electron is just ejected out of the metal but its velocity is zero. If υ is greater than υ0, then the difference in the energy (hυ - hυ0) is used in giving kinetic energy to the photoelectron.
            If mass of a photoelectron is m, and its velocity is v, then 


                             
           This is Einstein's Photoelectric equation.

Explanation of Laws of Photoemission
           If we assume that one photoelectron is ejected by one photon, we observe that the number of photoelectrons emitted per second from a metal depends on the number of photons incident on it per second. The number of photons incident per second on a surface depends on 'intensity of light'.
           Thus, the rate of emission of photoelectrons depends on the intensity of incident light (I Law).
          Since hυ0 is constant for a given metal, we find that from Einstein's Photoelectric equation.
          The kinetic energy of the photoelectron


           
          Thus, the kinetic energy of the photoelectron depends on υ, the frequency of the incident light (II Law).

 Einstein's Photoelectric equation was experimentally verified by 'Millikan' in 1916, for which he was awarded Nobel Prize in Physics for the year 1923.
Photoelectric cell
           It is an arrangment to convert light energy into electric energy (fig 2). This cell consists of a glass on Quartz bulb which is evacuated. It further consists a semi - cylindrical silver plate 'B' coated with Potassium or Cesium known as 'emitter' and a wire A of Platinum or Nickel called the collector (anode) fixed along the axis of the cylinder. Positive potential is applied to the collector A and the Photo - electron emitted from the cylinder B are attracted by it.
            The Photo - current produced in the circuit is detected by the Galvanometer 'G'.
            For quantitative work, the bulb is exhausted to the highest degree of vacuum. The sensitivity in this case is less. If however, the cell is filled with an inert gas Argon at low pressure, the sensitivity is increased.

Uses are so many!!

1. Photocells are used for 'automatic switching on and off of street lights.
2. Used in light meter in cameras to measure intensity of light.
3. Used in automatic closing and opening of doors.
4. Used in counting devices which counts every obstruction for light beam. Thus photocells count the number of persons entering into a building.
5. Used in fire alarms. When the light from a fire falls on the photocell, the current developed in the circuit makes the siren to produce alarm.
6. Used in burglar alarm utlraviolet rays are made to fall on the photocell continuously. A person entering into the room obstruct these rays, and electric bell rings.
7. Used to produce sound from the films in cinema theatres.
8. Used for scanning and telecasting in TV cameras.

9. Used for controlling the temperature in furnaces.
10. Used to find defects like holes and vacancies in metal sheets.
11. Used in complexion meters (colour identification) and traffic regulators.
Dual nature of matter
            In order to explain the phenomenon like interference, diffraction and polarisation by light, radiation is assumed to be possessing wave like particles.
   But phenomena like 'Photo - electric effect' which can be explained on the basis of Quantum theory has to assume that radiation behaves like a particle (Quanta or Photon) when it interacts with matter. Thus, with the introduction of Quantum theory physicists were obliged to admit a dual nature, Wave and a Particle for radiant energy.
           A similar situation arose with matter, when in 1924, the French Physicist Louis De Broglie put forward the bold suggestion that matter like radiation has dual nature i.e., matter which is ordinarily considered as made up of discrete particles like molecules, atoms, protons, neutrons and electrons may exhibit wave like properties under appropriate conditions. These waves are called Matter Waves.

De Broglie's Hypothesis
            While developing a theory of radiation in terms of light quanta on photons De Broglie was lead to the new conception of matter waves by the following considerations.
i) Nature loves symmetry: According to this principle matter and radiation (energy) the two fundamental forms in which nature manifests must be mutually symmetrical, as radiation exhibits dual nature wave and particle, matter also might possess the same dual nature.
ii) Analogy between mechanics and optics: While explaining the hydrogen spectrum by Bohr's theory, electron is assumed to move round the nucleus in orbits which are characterized by periodicity. Thus he found there is a close analogy between mechanics and optics. Mechanics dealing with particles is analogous to geometrical optics for light rays.
           Reflections, such as above led De Broglie to make a bold suggestion in his doctoral thesis in 1929. In his thesis he explained that there was a close connection between waves and corpuscles (particles) not only in case of radiation but also in case of matter.

MATTER WAVES 
            A moving particle always got a wave associated with it and the particle is controlled by the wave just as photon is controlled by waves. To study the path of a beam of monochromatic radiation, wave theory is used where as to calculate the amount of energy associated with the same beam, we use the photon or Quantum theory.
            Similarly, the electrons are particles i.e., their charge, mass and energy are observable in particle form. But in order to find the path of a beam of electrons and how it is reflected by objects, we must treat it as though it is a wave. It is to be noted that the energy is carried by the electrons and not by the waves associated with them.
De Broglie's wavelength (λ) 
             A photon of frequency υ is considered to be having energy E = hυ (according to Planck's theory), h - Planck constant.
            According to the theory of 'Relativity' E = mc2, where m is the mass of the photon and 'c' is the velocity of the photon.
              E = mc2 = hυ

 In a similar way De Broglie said that a particle of mass m, moving with a velocity v should be associated with wavelength λ such that


             
         λ is called De Broglie's wavelength and mv = p, the momentum of the particle.
Wavelength of Electron:
         If an electron is moving with a velocity 107 ms-1 in a discharge tube, it has a wavelength equal to


 Which is the order of the wavelength of X - rays.
           Here 6.62 × 10-34 = h, the Planck's constant.
           9.1 × 10-31 kg = m, the mass of electron.
 The accuracy of Louis de Broglie's suggestion of matter waves was verified experimentally by Davission and Germer in 1927, by their diffraction experiments with electrons (Diffraction shows the wave nature).
De Broglie's equation - significance


         
i) De Broglie wavelegnth (λ) of a particle is inversely proportional to the mass (m) of the particle. Particle of lesser mass will have greater wavelength.
ii) De Broglie wavelength of a particle (λ) is inversely proportional to the velocity (v) of the particle. Smaller is the velocity of the particle, greater will be its wavelength.
     Experimental demonstration of wave nature of electron
     Davisson and Germer Experiment

         The first experimental proof of the 'wave nature of electron' was demonstrated in 1927 by two American physicists C.J. Davisson and L.H. Germer.
         The basis of their experiment was that since the wavelength of an electron is of the order of spacing of atoms in a crystal, there should be 'Diffraction effects' when a beam of electrons when incident on a crystal.
         Experimental arrangement 
         Figure - 3 below shows the experimental arrangement made by Davisson and Germer.

         By a potential difference between anode A and cathode C, the electrons from hot filament F are accelerated. The narrow beam of electrons I emerging from the anode A is allowed to fall on the surface of a nickel crystal 'Ni'. The electrons, acting like waves, are scattered in all directions by the atoms in the crystal. By allowing the scattered electron beam to enter into a detector 'D', the intensity of electron beam 'S' scattered in a given direction is measured. Detector D is arranged on a graduated circular scale 'CS' that it can be rotated around the crystal. A sensitive galvanometer is connected to the detector D and the deflection of galvanometer is directly proportional to the intensity of electron beam entering the detector.
         The detector D is rotated to different positions on the circular scale and the intensity of the scattered beam is measured for different values of scattering angle  (i.e., angle between the incident beam and the scattered beam).
Observation
         When the electron beam is incident normally on the surface of the nickel crystal, the electrons are scattered in all directions which show that they are acting like waves. The intensity of scattered beam depends on the scattering angle , which is the angle between the incident and scattered beam.

        It is found that when Φ = 50° and V = 54 volts, the intensity of scattered beam becomes maximum. However, as the accelerating potential is increased, the intensity of scattered beam starts decreasing and at about 68 volts, the intensity almost comes down to zero.

           Fig-4 above shows the polar graphs i.e. the graph between scattering angle Φ and the intensity of scattered electron beam. The intensity of the electron beam in a given direction is proportional to the distance of the curve from the origin 'O'. The appearence of a bump (lump) (from the graphs) in a particular direction is due to the constructive interference of electrons scattered from different layers of regularly spaced atoms of the crystal. This confirms the wave nature of electron.
            G.P.Thomson (son of J.J.Thomson) was awarded 'Nobel Prize' in 1937 along with Davisson for their experiments of uncertainity is certain diffraction of electrons by crystals.
Heisenberg's Uncertainity Principle
       If we see Taj Mahal in full moon night and in dark night when no one is looking will it be different? If we look at electrons in motion, will there be any difference in their behaviour? - Quantum theory says yes, there will be a difference. From the vast depths of knowledge in Quantum theory, a jewel called 'Uncertainity Principle' was brought out by a great scientific visionary by name 'Werner Heisenberg', a Jerman physicist. Heisenberg who learnt lessons in mountaineering in childhood later studied physics and taken Quantum theory to greatest heights of scientific thought.

Heisenberg, whom scientific world praises as 'saint among scientists' is a path finder in prescribing limits to 'accuracy'. The matter - wave picture is clearly explained in the Heisenberg's 'Uncertainity Principle'.
According to this principle
         Statement: "The position and momentum of an electron (or any other particle) cannot be measured, (rather it is impossible) at the same time exactly".
         There is always some uncertainity '∆x' in the specification of position and some uncertainity "∆P" in the specification of momentum.

Explanation of the principle:
       The exact position and the exact velocity (rather momentum) of any subatomic particle cannot be found exactly at the same time.
       For example to find out where exactly an electron revolving around the nucleus with certain velocity in an orbit is, we have to focus a light particle i.e., a photon on it. Since electron and photon are particles of same size, when they collide the position of the electron changes.

        Similarly, if we try to stop the electron in the orbit and try to find its position, the energy in the photon will be absorbed by the electron and its velocity (momentum) changes. It means if we know the position of an electron, its velocity is not known. Similarly if we know the velocity of an electron, its position is not known. Thus the behaviour of electron is always uncertain. In fact subatomic particles and the whole universe (or nature) with which it was, made depends on 'Uncertainity'.
         The uncertainity in the universe is not due to the inaccuracy in our observation or the lack of precession in the apparatus we use for observation. Uncertainity is certain in the universe.
         When we are looking at Taj Mahal in moonlight or in dark night, 'Uncertainity' will be there which is so small (almost negligible) and hence we cannot observe it.
         Moreover, when compared to the size of the monument (Taj) the size of the photons focussed on it during night is almost negligible.

Conclusion:
Style of Physicists!

Dual Nature: 'Matter' and 'energy' are inter convertible, according to the equation
E = mc2 derived by Albert Einstein, the icon of acheivements of 20th century science.
        'Matter' and 'Energy' have dual nature i.e. they can be either in the form of 'waves' or 'particles'. Some one asked the famous Quantum Physicist Erwin Schrodinger how it is possible? He answered depending on time and circumstances they behave like that. The person further asked him sarcastically professor, do you mean to say that on Monday, Tuesday, and Wednesday they behave like 'Waves' and on Thursday, Friday and Saturday (Sunday being holiday) they behave like 'Particles'? Please tell me clearly wheather matter and Energy' will be in the form of 'waves' or 'particles'.
           For which the great scientist answered (laughing) - Matter and energy will not be either in the form of 'waves' or 'particles' but they will be in the form of "WAVICLES"!!
WAVE + PARTICLE → WAVICLE!!
 The great book 'What is life?' written by Erwin Schrodinger inspired 'Watson' and 'Crick' (Biologists) to discover 'DNA molecule' !!

Uncertainity Principle:
          According to uncertainity principle in Quantum Theory there is no place for cause and effect in nature. Albert Einstein who beleived in the 'certainity' in the universe created by God worried and disagreed with this principle and said "God is not playing dice".

Which means in the game of dice, there is no 'certainity' and every thing depends on chance.
          For Einstein's statement, Niels Bohr, the architect of 'Atomic structure basing on Quantum theory' and thus beleived in uncertainity principle reacted this.
          "Who is Einstein?, to ask God to play dice?
            It is very interesting to know after so many years, the famous living cosmologist 'Stephen Hawking', continuing the discussion on uncertainity principle making the statement that "God not only play dice but also throw them into space in such a way that he may not be able to find them".
          The famous book 'A Brief History of Time' by Stephen Hawking should be read by every student of science (especially physics student) to enjoy the beauty of the Universe!!

Posted Date : 04-11-2020

గమనిక : ప్రతిభ.ఈనాడు.నెట్‌లో కనిపించే వ్యాపార ప్రకటనలు వివిధ దేశాల్లోని వ్యాపారులు, సంస్థల నుంచి వస్తాయి. మరి కొన్ని ప్రకటనలు పాఠకుల అభిరుచి మేరకు కృత్రిమ మేధస్సు సాంకేతికత సాయంతో ప్రదర్శితమవుతుంటాయి. ఆ ప్రకటనల్లోని ఉత్పత్తులను లేదా సేవలను పాఠకులు స్వయంగా విచారించుకొని, జాగ్రత్తగా పరిశీలించి కొనుక్కోవాలి లేదా వినియోగించుకోవాలి. వాటి నాణ్యత లేదా లోపాలతో ఈనాడు యాజమాన్యానికి ఎలాంటి సంబంధం లేదు. ఈ విషయంలో ఉత్తర ప్రత్యుత్తరాలకు, ఈ-మెయిల్స్ కి, ఇంకా ఇతర రూపాల్లో సమాచార మార్పిడికి తావు లేదు. ఫిర్యాదులు స్వీకరించడం కుదరదు. పాఠకులు గమనించి, సహకరించాలని మనవి.

Special Stories

More

విద్యా ఉద్యోగ సమాచారం

More
 

లేటెస్ట్ నోటిఫికేష‌న్స్‌