Photoelectric effect


Photoelectric effect ? 

The phenomenon of emission of electrons from the metal surface when light radiations of suitable frequency fall on them. The electrons which are emitted as a result of light radiations are called photoelectrons and the current which is produced is called Photo electric current . 

Photoelectric effect


First, it was Hertz who in 1997 observed that the ultra violet light falling on the electrodes of a spark gap caused the sparks to jump greater distances than when the gap was not exposed to the ultra violet light. After this , Hallwachs  and his associates in 1888 , discovered that the negativity charged insulated zinc plate lost its charge when exposed to ultra violet light. Later on,  Leonard and his associates explained that the electrons are emitted by the zinc plate when exposed to ultra violet light and hence the zinc plate loses charge. Thus  it was concluded that the electrons are emitted by metal surfaces when exposed to ultra violet light. 

some metals show Photoelectric effect when ultra violet light falls on them and others show photo electric effect when visible light falls on them. If we consider metals like lithium, sodium, potassium they show photo electric effect when visible light falls on them and if we consider metals like zinc, magnesium etc when ultra violet light falls on them. 

Photoelectric effect


Electromagnetic radiations like light waves, radio waves etc. carry energy in the form packets or bundles which are known as photons or Quanta. 

Features of photons:

1) photons travel in straight line. 

2) The energy of photon is given by; 

                    E = hâ±±

                     E = h  c/λ

  3) All photons travel with the same speed equal to the speed of light. 

4)  Photons are electrically neutral. 

5)  The energy of photon is usually expressed in electron-volt, where one electron volt is equal to
     1.6 × 10 ̄¹⁹

6)  Photons are not deflected by the electric and magnetic fields because they are electrically neutral. 

7)  The momentum of photon is given by;

                      mc = h/λ

8) The mass of photon is given by; 

m = h / c λ

9) From the theory of relativity; 

          m₀ = m √1-V²/c²

    In case of photon , V ≈ C .

⇒     m₀ = 0 

   Hence , photons have zero rest mass and cannot exist at rest .

Photoelectric effect ,Photoelectric effect study
Photoelectric effect

Photoelectric effect

Experimental study of photoelectric current :

The apparatus used for the experimental study of photon electric effect is shown in figure... 

It consists of an evacuated quartz tube in which two plates named as P and Q called as cathode and anode are fitted. 

The plate Q can be given a desired positive or negative potent, using the potential dividing arrangement as shown in the figure. 

Voltmeter V,  is used to measure the potential difference between the plates P and Q where as the milliammeter (mA)  is used to measure the Photoelectric current in the circuit. 

When light rays of Suitable frequency are allowed to fall on the plate P which is having negative potential ,the electrons start to emit , these emitted electrons start to travel towards plate Q which is at positive potential. Hence, in this way photo electric current starts to flow through the circuit.

Photoelectric effect

A) Effect o0f intensity of incident light :

Give the plate Q, some definite value of positive potential. Now allow the light of some intensity to fall on the plate P. Due to this process, photoelectrons start to emit. These emitted photoelectrons Start to travel towards the plate Q and hence photo electric current starts to flow through the circuit.

If we increase the intensity of incident light falling on the plate P, we note that the Photoelectric current also increases and so on. When we draw graph between the intensity of incident light and photo electric current , note that we get a straight line. In conclusion, I would like to admit that the number of electrons emitted per second from the plate is directly proportional to the intensity of
 incident light.

Photoelectric effect 

B) Effect of potential:

Allow the light of some fixed intensity ( say I₁) to fall on the plate P. When the potential of the plate Q is at 0 volt , a little current that is equal to ' 0 k ' flows through the circuit. Now when the potential of the plate Q is made positive,  the  photo electric current increases till it reaches at maximum value. The maximum value of the current is called the saturation current. when the potential of the plate Q is made negative,  the photo electric current decreases sharply to zero. This potential at which Photo Electric current becomes zero is called the Stopping potential I. e, V₀ .

Hence,  stopping potential is defined as the minimum value of negative potential of plate Q, at which photo electric current becomes zero. It is also called as Cut-off potential.

If the experiment is repeated with the incident light of higher intensity, say I₂ ,then we note that the value of saturation current is more whereas stopping potential remains same. 

Thus we conclude that stopping potential does not depend upon the intensity of incident light. 

Also,  if m be the mass of photo electron and Vmax be it's maximum velocity then,  

Max. Kinetic energy of photo electron = 1/2 mV² max  ..................................... (1)

If the charge of photo electron is e and stopping potential of photo electron is V₀ ,then,  

work done to stop the photo electron = eV₀   ............................................ (2) 

from equations 1 and 2 , we get ; 

1/2mV² max = e V ₀ 

Thus, in the conclusion it is clear that the maximum velocity of emitted photo electrons does not depend upon the intensity of incident light .

Photoelectric effect

C) Effect of frequency:

Light of different wavelengths is allowed to fall on the plate P and the resulting photo electric current is measured by the micro-ammeter .It is found that whatever may be the applied potential difference or the intensity of light, the photo electric current starts only if the frequency of the incident light is more than a minimum specific value which is different for the different photo sensitive materials. This minimum frequency which is very essential for the photo - electric emission to commence is called a threshold frequency.

Threshold frequency depends upon the material and the nature of the surface of the material. For most materials,  this frequency lies in the ultra violet region.

This article provides you information about the Photoelectric effect ,its experimental study ,definition and effects of frequency ,potential and the intensity of incident light .


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