Dual nature
Electron emission :
The free electron can leave the metal surface only if an external energy is supplied to
overcome the surface barrier force
This process of emission of an electron from the metal surface is called electron emission
The minimum energy required to free an electron completely and leave the metal
surface is called as the work function of the metal (φ )
o
φ is measured in electron volt (eV)
o
-19
1 e V = 1.6 x 10 C x 1V
-19
1 e V = 1.6 x 10 J
Thermionic emission: The process of emission of electrons by heating the metal .
The emitted electrons here are called thermions
Field emission: The process of emission of electrons when a strong electric field is applied
across the metal
Photoelectric emission (Photo electric effect): The process of emission of electrons when
light of a suitable frequency is incident on a metal surface
The minimum frequency of incident light required to emit the electrons from the surface is
called the threshold frequency
The emitted electrons are called photo electrons
Threshold frequency depends on the metallic surface and is different for different metals
The current that flows in the circuit due to the photo electrons is called the photo
electric current
UV light causes photoelectric emission for any metal
LHSS PHYSICS ’25 XII (LD) 1
,Effect of intensity of incident light on the photoelectric current :
The photoelectric current is directly proportional to the intensity of the incident light ,
provided the frequency of the incident light is greater than the threshold frequency of the
metal surface
Effect of the potential applied on the photoelectric current :
Keeping the frequency and intensity of the incident light fixed, it was noticed that,
The photoelectric current increases with increase in positive potential of collector plate
At one stage, for a certain positive potential of collector plate , the photoelectric current
becomes maximum or saturates. This maximum value of photoelectric current is called
saturation current
Photoelectric current is rapidly decreased to zero with increasing negative potential of plate A
(collector plate)
For this the polarity of the plates are reversed and the electrons are repelled
The minimum negative potential required for which the photoelectric current should
become zero is called cut off or stopping potential (−𝐕𝐎 )
Keeping the frequency of the incident light fixed, it was noticed that,
2 1
1
S
P
S
P
(S )
(I – Intensity of light )
• Stopping potential is independent of the intensity of light
• Saturation current depends on the intensity of the incident light
LHSS PHYSICS ’25 XII (LD) 2
, Effect of frequency of the incident light :
Keeping the intensity of the incident light fixed, it was noticed that,
P
S
P
( f – Frequency of light )
• The value of saturation current is independent of frequency of the incident light
• The value of stopping potential depends upon the frequency of the incident light
LHSS (PHY 2 2 2 21 XII) D S
Photoelectric effect is instantaneous :
Since photoelectrons are emitted as soon as light of a frequency greater than the
threshold frequency is incident on a metal surface that is why the process is
instantaneous
Planck proposed the assumption that light energy given out exist in discrete quanta or energy
packets , which are proportional to the frequency.
Therefore he concluded that , electromagnetic radiation (light) is quantized i.e occurs in finite
"bundles" of energy which are called photons
The energy of each photon , E = hf
In general , E = N hf
where – h : Planck’s constant
f : frequency of radiation
N : Number of photons
- 34
h = 6.634 x 10 Js
Some properties of photons :
▪ Each photon has energy and momentum
▪ They have no charge , hence are electrically neutral and are not deflected by electric
or magnetic fields
▪ The energy of a photon is dependent on the frequency of light
▪ A photon moves at the speed of light in vacuum
LHSS PHYSICS ’25 XII (LD) 3
Electron emission :
The free electron can leave the metal surface only if an external energy is supplied to
overcome the surface barrier force
This process of emission of an electron from the metal surface is called electron emission
The minimum energy required to free an electron completely and leave the metal
surface is called as the work function of the metal (φ )
o
φ is measured in electron volt (eV)
o
-19
1 e V = 1.6 x 10 C x 1V
-19
1 e V = 1.6 x 10 J
Thermionic emission: The process of emission of electrons by heating the metal .
The emitted electrons here are called thermions
Field emission: The process of emission of electrons when a strong electric field is applied
across the metal
Photoelectric emission (Photo electric effect): The process of emission of electrons when
light of a suitable frequency is incident on a metal surface
The minimum frequency of incident light required to emit the electrons from the surface is
called the threshold frequency
The emitted electrons are called photo electrons
Threshold frequency depends on the metallic surface and is different for different metals
The current that flows in the circuit due to the photo electrons is called the photo
electric current
UV light causes photoelectric emission for any metal
LHSS PHYSICS ’25 XII (LD) 1
,Effect of intensity of incident light on the photoelectric current :
The photoelectric current is directly proportional to the intensity of the incident light ,
provided the frequency of the incident light is greater than the threshold frequency of the
metal surface
Effect of the potential applied on the photoelectric current :
Keeping the frequency and intensity of the incident light fixed, it was noticed that,
The photoelectric current increases with increase in positive potential of collector plate
At one stage, for a certain positive potential of collector plate , the photoelectric current
becomes maximum or saturates. This maximum value of photoelectric current is called
saturation current
Photoelectric current is rapidly decreased to zero with increasing negative potential of plate A
(collector plate)
For this the polarity of the plates are reversed and the electrons are repelled
The minimum negative potential required for which the photoelectric current should
become zero is called cut off or stopping potential (−𝐕𝐎 )
Keeping the frequency of the incident light fixed, it was noticed that,
2 1
1
S
P
S
P
(S )
(I – Intensity of light )
• Stopping potential is independent of the intensity of light
• Saturation current depends on the intensity of the incident light
LHSS PHYSICS ’25 XII (LD) 2
, Effect of frequency of the incident light :
Keeping the intensity of the incident light fixed, it was noticed that,
P
S
P
( f – Frequency of light )
• The value of saturation current is independent of frequency of the incident light
• The value of stopping potential depends upon the frequency of the incident light
LHSS (PHY 2 2 2 21 XII) D S
Photoelectric effect is instantaneous :
Since photoelectrons are emitted as soon as light of a frequency greater than the
threshold frequency is incident on a metal surface that is why the process is
instantaneous
Planck proposed the assumption that light energy given out exist in discrete quanta or energy
packets , which are proportional to the frequency.
Therefore he concluded that , electromagnetic radiation (light) is quantized i.e occurs in finite
"bundles" of energy which are called photons
The energy of each photon , E = hf
In general , E = N hf
where – h : Planck’s constant
f : frequency of radiation
N : Number of photons
- 34
h = 6.634 x 10 Js
Some properties of photons :
▪ Each photon has energy and momentum
▪ They have no charge , hence are electrically neutral and are not deflected by electric
or magnetic fields
▪ The energy of a photon is dependent on the frequency of light
▪ A photon moves at the speed of light in vacuum
LHSS PHYSICS ’25 XII (LD) 3
