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Dual Nature of Matter and Radiation Class 12 Physics

                                                                                                                                                   


Dual Nature of Matter and Radiation

Introduction

  1. The discovery of phenomena like interference, diffraction and polarization established that wave of nature.

  2. Maxwell’s equations of electromagnetism and Hertz experiment on production and detection of electromagnetic waves in 1886 strongly supported the concept of wave nature of light .

  3. The discovery of cathode rays by William Crookes in 1879, of X-rays by Roentgen in 1895 and of electrons by J.J. Thomson in 1897 were very important as these discoveries helped to understand the atomic structure.

  4. J.J. Thomson predicted that the cathode rays were nothing but a stream of fast  moving electrons.




  1. J.J.Thomson , using the  mass spectrograph, experimentally determined the velocity and the specific charge( charge / mass) of cathode rays.

  2. He concluded that the speed of cathode rays ranging from 0.1 to 0.2 times of speed of light and the accepted value of specific charge of cathode rays is 1.76 x 10 power 11 coulomb / kg which is independent of

(a) nature of material used as cathode and 

(b) nature of gas introduced in the discharge tube.

This shows that particles are universally alike.

  1. In 1887 ,it was found that if certain metals were irradiated by ultraviolet light, electrons were emitted having small velocities . It was also found that if certain metals are heated to high temperature, the electrons emitted. These study led scientists to think that electrons are fundamental particles and are universal constituent of matter .

  2. Millikan measured experimentally that charge on an oil droplet and he predicted that charge on an oil droplet is always an integral multiple of elementary  charge 1.602 x{"code":"$10^{-19}$","id":"1","aid":null,"backgroundColor":"#ffffff","backgroundColorModified":false,"type":"$","font":{"family":"Arial","size":11,"color":"#000000"},"ts":1622770082277,"cs":"1Cd8mGwzFQe4PAGiVl6ljQ==","size":{"width":36,"height":14}} coulomb, which is the integral multiple of charge of an electron.

This shows that the charge on a body is quantized. 

  1. The knowledge of the values of charge (e) and specific charge(e/m) of electron , helped to determine the mass of the electron,


m= e/{"font":{"size":11,"color":"#000000","family":"Arial"},"aid":null,"id":"2","type":"$","code":"$\\frac{e}{m}$","backgroundColorModified":false,"backgroundColor":"#ffffff","ts":1622770200905,"cs":"TjJV2sW/zPMgQklJRrO1xA==","size":{"width":13,"height":17}} = {"backgroundColor":"#ffffff","code":"$\\frac{1.602}{1.76}$","type":"$","backgroundColorModified":false,"aid":null,"font":{"family":"Arial","color":"#000000","size":11},"id":"3","ts":1622770438598,"cs":"L8+mEnAFfK4SlCIfvAw9hA==","size":{"width":30,"height":20}}{"backgroundColor":"#ffffff","code":"$\\times\\frac{10^{-19}}{10^{11}}$","type":"$","aid":null,"id":"4","font":{"family":"Arial","size":14,"color":"#000000"},"backgroundColorModified":false,"ts":1622770721822,"cs":"pOx0wr0VB3GHswdK6Ukl3Q==","size":{"width":56,"height":32}}= 9.11{"type":"$","aid":null,"font":{"family":"Arial","color":"#000000","size":30},"backgroundColorModified":false,"backgroundColor":"#ffffff","id":"5","code":"$\\times$","ts":1622770906216,"cs":"ohmOAUjXLoDj0xNYYATlnA==","size":{"width":22,"height":21}}{"backgroundColor":"#ffffff","backgroundColorModified":false,"id":"6","aid":null,"font":{"color":"#000000","family":"Arial","size":30},"code":"$10^{-31}$","type":"$","ts":1622770948314,"cs":"SlLxedcH7XTpkbfGDdkEqQ==","size":{"width":98,"height":41}} kg


  1. The discovery of Photoelectric effect by Hertz,  Compton effect by Compton, Stark  effect by Stark were explained by Quantum theory of light. 

  2. Planck's Quantum theory : According to Quantum theory of light,” The light consists of the packets of energy which travel in a straight line with a speed of light. Each packet of energy is called for photon or quantum of light .”

The energy of photon is given by


E = h {"code":"$\\nu$","backgroundColor":"#ffffff","font":{"size":11,"family":"Arial","color":"#000000"},"id":"8","aid":null,"backgroundColorModified":false,"type":"$","ts":1622771306644,"cs":"XKgCkW2sHU/l1YFIYwP5vg==","size":{"width":8,"height":6}} =  {"code":"$\\frac{hc}{\\lambda}$","backgroundColor":"#ffffff","backgroundColorModified":false,"font":{"color":"#000000","family":"Arial","size":18},"aid":null,"id":"7","type":"$","ts":1622771145357,"cs":"u4azPtR49Wf/IQv8k1l8sg==","size":{"width":24,"height":34}}


Here,

E = energy of photon

{"backgroundColorModified":false,"backgroundColor":"#ffffff","aid":null,"id":"9","code":"$\\nu$","font":{"size":11,"family":"Arial","color":"#000000"},"type":"$","ts":1622771372224,"cs":"0N/sWe7ZDIG6/CTKoa33Mg==","size":{"width":8,"height":6}} = frequency of light/ radiation

C = speed of light

{"backgroundColor":"#ffffff","code":"$\\lambda$","type":"$","font":{"size":11,"family":"Arial","color":"#000000"},"backgroundColorModified":false,"id":"10","aid":null,"ts":1622771437108,"cs":"LSYq6mgBPzfbbJQMJUWzBg==","size":{"width":8,"height":12}} = wavelength  of light

Thus, particle nature of light was established

  1. As some phenomena of light were explained by wave theory of light and some by particle nature of light, hence it was concluded that light is of dual nature.



 


Free Electrons in Metals

In metals, the electrons in the outer shell called Valence Electrons of atoms are loosely bound. They are quite free to move easily on the metal surface. Such loosely bound electrons are called free electrons.


Work Function

  1. The minimum energy required by an electron to just escape from the metal surface so as to overcome the restraining force at the surface is called work function of a metal .

  2. The work function of a metal is generally denoted  by {"backgroundColor":"#ffffff","aid":null,"type":"$","backgroundColorModified":false,"code":"$\\phi_{0}$","font":{"family":"Arial","size":18,"color":"#000000"},"id":"11","ts":1622771549088,"cs":"wggSjk5QelkaxDV2CkTlng==","size":{"width":24,"height":24}} or {"id":"12","aid":null,"font":{"size":14,"family":"Arial","color":"#000000"},"code":"$W_{0}$","backgroundColorModified":false,"backgroundColor":"#ffffff","type":"$","ts":1622771644482,"cs":"26icqxvaoEv9NuzRw0eJJg==","size":{"width":26,"height":17}}

  3. The work function is generally given by


       {"backgroundColorModified":false,"backgroundColor":"#ffffff","code":"$\\phi_{0}$","font":{"color":"#000000","size":14,"family":"Arial"},"id":"13","type":"$","aid":null,"ts":1622771757668,"cs":"AKE5EE38ZfDj1Kcp21p0zA==","size":{"width":18,"height":18}} = h{"code":"$\\nu_{0}$","backgroundColorModified":false,"id":"14","backgroundColor":"#ffffff","aid":null,"font":{"color":"#000000","family":"Arial","size":11},"type":"$","ts":1622771842983,"cs":"4xOl1kXLH2G9vuTBJk6drw==","size":{"width":12,"height":9}} 

Here,

         {"backgroundColor":"#ffffff","code":"$\\phi_{0}$","id":"15","font":{"size":14,"family":"Arial","color":"#000000"},"backgroundColorModified":false,"type":"$","aid":null,"ts":1622771959501,"cs":"k9+Hre2QnSEtXMzcA3R+sQ==","size":{"width":18,"height":18}} = Work function 

           h= Planck’s constant

             {"aid":null,"backgroundColorModified":false,"backgroundColor":"#ffffff","code":"$\\nu_{0}$","type":"$","font":{"size":11,"family":"Arial","color":"#000000"},"id":"16","ts":1622772122752,"cs":"qAPOa4QgT62h1oAyGl5Npw==","size":{"width":12,"height":9}}= Threshold frequency


  1. (a) The work function of a metal is measured in electron volt (eV).. 

(b) One electron volt

 When an electron is accelerated between two points having a potential difference of .One volt, then energy gained by an electron called one electron volt.


1 e V = 1 electron x 1 volt

        = 1.60 {"type":"$","code":"$\\times$","id":"17","backgroundColor":"#ffffff","aid":null,"backgroundColorModified":false,"font":{"family":"Arial","color":"#000000","size":11},"ts":1622772381388,"cs":"FlC5xlRkOWt1WH/+wewrJA==","size":{"width":8,"height":8}}{"font":{"size":11,"color":"#000000","family":"Arial"},"code":"$10^{-19}$","backgroundColor":"#ffffff","id":"18","aid":null,"type":"$","backgroundColorModified":false,"ts":1622772459593,"cs":"GoWZnx2swaQyaYcKEVtmGw==","size":{"width":36,"height":14}} C X 1 volt

        = 1.60 {"id":"20","type":"$","aid":null,"font":{"family":"Arial","size":11,"color":"#000000"},"backgroundColor":"#ffffff","code":"$\\times$","backgroundColorModified":false,"ts":1622772743391,"cs":"BhLSiD3wMYcOcDkRPDjzIg==","size":{"width":8,"height":8}}{"type":"$","aid":null,"id":"21","font":{"size":11,"color":"#000000","family":"Arial"},"backgroundColor":"#ffffff","backgroundColorModified":false,"code":"$10^{-19}$","ts":1622772788958,"cs":"zgwCTF31f/bQbpKhzjK/Fg==","size":{"width":36,"height":14}} joule

1 M e V= 1.60 {"font":{"color":"#000000","family":"Arial","size":11},"type":"$","code":"$\\times$","backgroundColor":"#ffffff","aid":null,"backgroundColorModified":false,"id":"22","ts":1622772949831,"cs":"B4TVqnqECHvV/biZxSspkw==","size":{"width":8,"height":8}}{"backgroundColorModified":false,"id":"23","type":"$","aid":null,"font":{"family":"Arial","color":"#000000","size":11},"code":"$10^{-13}$","backgroundColor":"#ffffff","ts":1622772981284,"cs":"kcajMFZ4btRqdykvCjiX9g==","size":{"width":36,"height":14}} J

(c) Electron volt is a unit of energy generally used in atomic physics and nuclear physics.

  1. The work function is lowest for cesium and highest for platinum .

  2. Alkali metals like Lithium, Sodium, Potassium,Cesium e.t.c. show Photoelectric effect with visible light.

  3. Metals like zinc, cadmium, magnesium e.t.c.are sensitive only to ultraviolet light.


By Shivanand Choudhry

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