Skip to main content

Mirror formula | Class 10 science | Concave mirror | Convex mirror | Physics

Post a Comment

Hertz, Hallwacks and Lenards's Observations on Photoelectric Effect



 

Hertz, Hallwacks and Lenard's observation on Photoelectric effect


Hertz observation

  1. The phenomenon of photoelectric emission was discovered by Heinrich Hertz in 1887, when he was working with his electromagnetic wave experiment.

  2. When suitable radiations fall on a metal surface,some electrons near the surface observe enough energy from the incident radiation. Due to it, they are able to overcome the attraction of the positive ions in the material of the surface and escape to the surrounding space.



Hallwacks and Lenard’s observation

  1. Hallwack and Lenard made a detailed study of the Photoelectric effect during 1886 to 1902.

  2. They observed,” when ultraviolet radiation falls on an emitter plate that is positive plate electrons are emitted from it which are attracted towards the other metal plate called collector plate kept at positive potential.The flow of electrons through the evacuated glass tube results in the current flow in the external circuit.

 Thus light falling on the surface of emitter causes current in the external circuit

  1. They also observed that negatively charged particles were ejected out from the zinc plate under the action of ultraviolet radiations.

  2.  They also observed that there was no emission of electrons from the emitter plate if the frequency of the light was smaller than a certain minimum value called Threshold frequency.

 The value of Threshold frequency depends on the nature of material of the emitter plate.




Experimental  study of Photoelectric effect


Diagram



Here,

C = Cathode or emitter or negative electrode

A = Anode or plate or collector or positive electrode

W = Window

{"font":{"color":"#000000","size":11,"family":"Arial"},"type":"$","backgroundColorModified":false,"code":"$\\mu$","aid":null,"backgroundColor":"#ffffff","id":"2","ts":1622695125339,"cs":"hM6zk90QnhhddQCPzYFnqA==","size":{"width":8,"height":10}}A = Microammeter

V =  Voltmeter

B = Battery

K = Key


The schematic diagram of experimental arrangement is shown in figure to study  in detail.


Effect of intensity of the incident radiations

  1. Using the incident radiations of a fixed frequency and keeping the plate A at a fixed suitable voltage, it is found that photoelectric current varies linearly with the intensity of  the incident radiations as shown graphically in figure.

  2.  As the photoelectric current is directly proportional to the number of photoelectrons  emitted per second, so the number of photoelectrons emitted per second is directly proportional to the intensity of incident radiations.

  3. Graph







Effect of potential of plate A with respect to plate C

1. At a fixed frequency and fixed  intensity, it is found that photoelectric current increases gradually with increase in positive potential.

2. At one stage for a positive potential of plate A, the photoelectric current becomes maximum or saturate. This maximum value of photoelectric current is called saturation current.

3.The saturation current corresponds to the state when all photoelectrons emitted from cathode C  reach the anode plate A .

4. Now applying negative potential to A with respect to C, photoelectric current decreases. It is so because the photoelectrons emitted from C are repelled by negative potential of A and only  highly energetic photoelectrons are reaching the plate A .

5. By increasing negative potential of a gradually we find that photoelectric current decreases rapidly and it becomes zero at a certain value of negative potential {"type":"$","backgroundColor":"#ffffff","aid":null,"code":"$V_{0}$","id":"3","backgroundColorModified":false,"font":{"family":"Arial","color":"#000000","size":12},"ts":1622698267441,"cs":"s0jl3TPgilGFZOYLhUAFew==","size":{"width":16,"height":16}}on plate A. This negative potential of A is called stopping potential or cut off potential.

6. At this stage even the fastest electrons can not reach the plate A .

7. Stopping potential

The minimum negative potential(    ) given to plate A with respect to plate C  at which the photoelectric current becomes zero is called stopping potential or cut off potential.

If e be the charge on the photoelectron, then work done in stopping potential is given by


W = e.{"backgroundColor":"#ffffff","id":"4","aid":null,"code":"$V_{0}$","backgroundColorModified":false,"font":{"family":"Arial","color":"#000000","size":11},"type":"$","ts":1622699242607,"cs":"AHosy2F9yJrHAJOp9UvSgg==","size":{"width":13,"height":13}}                 ( W = Q.V )

And, Kinetic energy of electron = {"backgroundColorModified":false,"id":"11","backgroundColor":"#ffffff","type":"$","font":{"color":"#000000","family":"Arial","size":11},"code":"$\\binom{1}{2}$","aid":null,"ts":1622707107938,"cs":"5mvu3Cpt2YjhUxBAGrJRjg==","size":{"width":16,"height":21}}m{"code":"$v^{2}$","type":"$","font":{"size":11,"family":"Arial","color":"#000000"},"id":"12","aid":null,"backgroundColorModified":false,"backgroundColor":"#ffffff","ts":1622707189582,"cs":"XXvRijdqVuAspy5fSMQExw==","size":{"width":12,"height":13}}

Where,

            m= mass of electron

            V = maximum velocity of electron


Here electron is stopped by work done by potential of A

 Therefore, 

                    W =  Kinetic energy

                    e{"backgroundColor":"#ffffff","font":{"color":"#000000","family":"Arial","size":11},"id":"13","backgroundColorModified":false,"code":"$V_{0}$","type":"$","aid":null,"ts":1622707650391,"cs":"zKIC3w6o9ssu0tFFxWFMvA==","size":{"width":13,"height":13}} =  {"id":"16","type":"$","code":"$\\binom{1}{2}$","aid":null,"font":{"family":"Arial","color":"#000000","size":11},"backgroundColorModified":false,"backgroundColor":"#ffffff","ts":1622707987690,"cs":"D3bWF7aFQ5Ld14NTbtUwxA==","size":{"width":16,"height":21}}m {"font":{"size":11,"family":"Arial","color":"#000000"},"aid":null,"backgroundColor":"#ffffff","backgroundColorModified":false,"id":"18","type":"$","code":"$v^{2}$","ts":1622708155397,"cs":"9MyX8n3NHnqj3WOjPk+QQg==","size":{"width":12,"height":13}} = Kinetic energy


8.  Conclusion

(a) All photoelectrons emitted from metal plate C are not having the same kinetic energy.

(b) For the radiations of a given frequency and material of C,the value of the stopping potential is independent of the intensity of incident radiation as shown in the graph. 





Effect of frequency of incident radiations

  1. Taking radiations of different frequencies at constant intensity, we get the variations of type as shown in figure.

Graph



2.From the graph we find that 

  1. The value of stopping potential is different for different radiation of different frequencies. 

  2. The value of stopping potential is more negative for Radiation of higher incident frequency.

  3. The value of saturation current depends on the intensity of incident radiation but is independent of frequency of incident radiation. 

3. (a) If we plot a graph between stopping potential and the frequency of the incident radiation for two different metals A and B, we get the graph as shown in figure.


(b)From the graph find that 

(i) For a given photosensitive material, the stopping potential varies linearly with the frequency of incident radiation. 

(ii) For a given photosensitive material there is a certain minimum cut off frequency {"type":"$","id":"19","backgroundColorModified":false,"aid":null,"font":{"size":11,"color":"#000000","family":"Arial"},"backgroundColor":"#ffffff","code":"$\\nu_{0}$","ts":1622718730683,"cs":"LdW5O0h+6JrWTyEk99JcbA==","size":{"width":12,"height":9}} called Threshold frequency for which the stopping potential is zero. 

(iii) The higher is the work function for a photosensitive material, the greater is the value of Threshold frequency. 

(iv) The intercepts on the potential axis = - {"backgroundColorModified":false,"font":{"color":"#000000","size":11,"family":"Arial"},"aid":null,"backgroundColor":"#ffffff","id":"20","type":"$","code":"$\\phi_{0}$","ts":1622719156449,"cs":"iSjeL7Mh7gl5NCW+DggI/w==","size":{"width":14,"height":14}}/e = - h{"aid":null,"type":"$","backgroundColorModified":false,"font":{"family":"Arial","color":"#000000","size":11},"backgroundColor":"#ffffff","id":"23","code":"$\\nu_{0}$","ts":1622719472628,"cs":"IkObEKZkK/3Sffjfyqmytw==","size":{"width":12,"height":9}}/ e .


 Therefore work function = {"code":"$\\phi_{0}$","font":{"color":"#000000","size":18,"family":"Arial"},"aid":null,"backgroundColor":"#ffffff","backgroundColorModified":false,"type":"$","id":"24","ts":1622719644859,"cs":"gDQdRiK7KfUdulqDubXGMg==","size":{"width":24,"height":24}} = e. magnitude of intercept of the potential axis.




Laws of photoelectric emission

  1. For a given metal and frequency of incident radiation, the number of photoelectrons ejected per second is directly proportional to the intensity of the incident light.

  2.   For a given metal, there exists a certain minimum frequency of the incident radiation below which no emission of photoelectrons takes place.This minimum frequency is called threshold frequency.

  3. Above the threshold frequency, the maximum kinetic energy of the emitted photoelectrons is independent of the intensity of the incident light but depends only upon the frequency or wavelength of the incident light.

  4. The photoelectric emission is an instantaneous process.

The time lag between the incidence of radiation and emission of photoelectrons is {"code":"$10^{-9}$","font":{"size":11,"family":"Arial","color":"#000000"},"id":"25","aid":null,"backgroundColorModified":false,"backgroundColor":"#ffffff","type":"$","ts":1622779903364,"cs":"fjjE4fmauDlXoq0M4yoXpA==","size":{"width":30,"height":14}}very small, less than even  second .





By Shivanand Choudhry


Dual Nature of Matter and Radiation Class 12 th Physics

Lecture-02


Website:

https://www.snphysicsexpert.com

Blog:

https://snphysicsexpert.blogspot.com

Facebook page:

https://www.facebook.com/snphysicsexpert

Facebook:

https://www.facebook.com/shivanand.chaudhary.5

Instagram:

https:www.instagram.com/shivanand.chaudhary.5

Twitter:

https://www.twitter.com/snchaudhary2014

LinkedIn:

https://www.linkedin.com/in/shivanand-chaudhary-7061028b

B.D.GURUKUL

Google page:

https://g.page/BDGurukul

Youtube Channel :

https://youtube.com/c/BDGURUKUL


#bdgurukul

#physics

#dualnatureofradiationandmatter

#dualnatureofradiation

#snphysicsexpert

#photoelectriceffect



Labels:

Comments

Post a Comment

If any student or reader has any doubt, comment me. I will reply within 24 hours.
Thanks

Popular posts from this blog

Dual Nature of Matter and Radiation Class 12 Physics

                                                                                                                                                    Dual Nature of Matter and Radiation Introduction The discovery of phenomena like interference, diffraction and polarization established that wave of nature. 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 . 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 importan...
Post a Comment
Read more

Light | Reflection of Light | Laws of reflection | Classs 10 physics

Light | Reflection of Light | Laws of reflection | Classs 10 physics Here we study class 10 physics,optics,light,reflection of light,laws of reflection,plane mirror,reflection by plane mirror,theory of light. Light 1. Light is a form of energy which enables us to see an object. 2.It is an electromagnetic wave. 3.Light waves are transverse in nature. 4.Light waves are electromagnetic radiation so they do not need a material medium to  travel. 5.It travels in vacuum as well as in material transparent medium. 6. The speed of light is 300000km/s in vacuum. 7.The speed of light in material medium is less than in vacuum. 8. Light travels in a  straight line. 9.Light is the fastest messenger in the universe. Theory of Light There are four theories of light. These are 1 . Newton's Corpus cular Theory of Light  : It explains corpuscles/particles nature of light.It proves rectillinear propagation of light, Reflection of light,Refraction of light. It does not prove interference of ...
Post a Comment
Read more