Chapter 3 photoelectric effect

SCOPE OF STUDY
5 main sub topics students should learn and understand in this
chapter are :
Effect of intensity and frequency of a light wave on the
photoelectrons produced
Photoelectric current against potential graph
Quantitative study of the equations, work function and
threshold frequency
Photon theory of light
Failure of wave optics in explaining the photoelectric effect

PHOTOELECTRIC EFFECT
DEFINITION
DEFINITION

It's been determined experimentally that
It's been determined experimentally that
when light shines on a metal surface, the
when light shines on a metal surface, the
surface emits electrons
surface emits electrons

Example : You can start a current in a circuit just by shining a light on
a metal plate.

Why do you think this happens?
Well, we were saying earlier that light is made up of
electromagnetic waves, and that the waves carry energy. So if a
wave
of light hit an electron in one of the atoms in the metal, it might
transfer enough energy to knock the electron out of its atom.

Example :

A metal plate, P and a small electrode, C are placed inside an
evacuated glass tube (photocell).
2 electrodes are connected to an ammeter and a source of emf.
When photocell is in dark, ammeter reads zero (I = 0A).
When light of sufficiently high frequency illuminates the plate, the
ammeter indicates the current following in the circuit.

How it works??
Imagining that electrons ejected from the plate by the impinging
radiation flow across the tube from the plate to the collector, C.
That electrons emit when light shines on a metal surface is consistent
with the electromagnetic (EM) wave theory of light : The electric field
of EM wave exert a force on electrons in the metal eject some of the
electrons.

PHOTOEMISSIVE
PHOTOEMISSIVE
A material that can exhibit the
A material that can exhibit the
photoelectric effect
photoelectric effect

PHOTOELECTRONS
PHOTOELECTRONS
The ejected electrons
The ejected electrons

Historically, light has sometimes been viewed as a particle rather
than a wave.
Einstein pointed out the wave theory and the photon theory of light
give different predictions of the photoelectric effects.
Two important properties of light wave are its intensity and its
frequency (or wavelength).
Effect of intensity and frequency of a light wave on the
photoelectron produced is described on wave theory predictions and
photon theory predictions.

Wave Theory Predictions
Wave Theory Predictions
If light is aawave, theory predicts:
If light is wave, theory predicts:
1. If the light intensity increase, the number of electrons
1. If the light intensity increase, the number of electrons
ejected and their maximum kinetic energy increase.
ejected and their maximum kinetic energy increase.
Because the higher intensity means aa greater electric field
Because the higher intensity means greater electric field
amplitude and the greater electric field should eject
amplitude and the greater electric field should eject
electrons with higher speed.
electrons with higher speed.
2. The frequency of light not affect the kinetic energy of the
2. The frequency of light not affect the kinetic energy of the
ejected electrons.
ejected electrons.

Photon Theory Predictions
Photon Theory Predictions
If light is particles, theory predicts:
If light is particles, theory predicts:
•• Increasing intensity increases number of electrons but not
Increasing intensity increases number of electrons but not
energy.
energy.
•• Above aa minimum energy required to break atomic bond,
Above
minimum energy required to break atomic bond,
kinetic energy will increase linearly with frequency.
kinetic energy will increase linearly with frequency.
•• There is aa cutoff frequency below which no electrons will be
There is cutoff frequency below which no electrons will be
emitted, regardless of intensity.
emitted, regardless of intensity.

Stopping Potential or Cutoff Potential, Vo

The negative potential of the plate 'C' at which the
photo electric current becomes zero. Stopping potential
is that value of retarding potential difference between
two plates which is just sufficient to halt the most
energetic photo electrons emitted.

*To determine the maximum kinetic energy, K max
*To determine the maximum kinetic energy, K max
from VO,,use the conservation of energy ::
from VO use the conservation of energy
Loss of kinetic energy = Gain in potential energy
Loss of kinetic energy = Gain in potential energy

K max = e VO
K max = e VO

If we draw the photo electric curve by plotting the photo electric
current 'I' verses the accelerating voltage 'V', the graph so obtained is
shown below.
Graph shows that there is a saturation current for different
intensities and even when V=0, there is some photo electric current io.
The curve shows that the stopping potential is independent of the
intensity of radiation.

I
At intensity III
At intensity II
At intensity I
i
Vo

V

Saturation current

If these curves are plotted for different frequencies V1 and V2 but
with same intensity, the curve shows the behavior as shown.

The saturation current depends upon intensity and not on frequency.
However, the stopping potential becomes more negative from (Vo)1
to (Vo)2 with the increase in frequency.

I
Constant intensity

Saturation
current
(Vo )2 (Vo )1

i

V

OTHER FUNDAMENTAL LAWS OF PHOTO
OTHER FUNDAMENTAL LAWS OF PHOTO
ELECTRIC EMISSION
ELECTRIC EMISSION


The no. of electrons emitted per second i.e. photo current is

proportional to the intensity of incident light.
 If frequency of incident radiation is below threshold
frequency, no photo electric emission will take place.
 The max. velocity or max. K.E of photoelectrons depends on
the frequency of radiation not on intensity. K.E. Increases with
the increase in frequency.
.

 The rate at which the electrons are emitted from a photo

cathode

is

independent

of

its

temperature.

This shows that photo electric effect is entirely different from
thermionic emission.
 For a given metal surface, stopping potential (Vo) is
directly proportional to frequency but independent of
intensity.

Work Function, WO
Work Function, WO
Minimum amount of energy which is necessary to
Minimum amount of energy which is necessary to
start photo electric emission
start photo electric emission
# Remember that :
1. It is a property of material.
2. Different materials have different values of work function.
3. Generally, elements with low I.P values have low work function
such as Li, Na, K, Rb, and Cs.

All the photon energy is transferred to the electron and the photon
All the photon energy is transferred to the electron and the photon
ceases to exist.
ceases to exist.
Electrons are held in the metal by attractive forces, some minimum
Electrons are held in the metal by attractive forces, some minimum
energy, WOOis required just to get an electron out through the surface.
energy, W is required just to get an electron out through the surface.
If hf < Woo ,, the photons will not have enough energy to eject any
If hf < W the photons will not have enough energy to eject any
electrons at all.
electrons at all.
If hf > Woo ,, electrons will be ejected and energy will be conserved
If hf > W electrons will be ejected and energy will be conserved
in the process. This will come out equation ::
in the process. This will come out equation
If the least bound electrons,
If the least bound electrons,

hf = K + W
hf = K + W

hf = Kmax + Woo
hf = Kmax + W

THRESHOLD FREQUENCY, ffo
THRESHOLD FREQUENCY, o

The minimum frequency of incident light which
can cause photo electric emission i.e. this
frequency is just able to eject electrons with out
giving them additional energy.

The particle theory assumes that an electron absorbs a single photon.
Plotting the kinetic energy vs. frequency:

This shows clear agreement with the photon theory, and not with
wave theory.
The maximum kinetic energy of ejected electrons increases linearly
with the frequency of incident light.

Kmax = hf - WO
No electrons are emitted if f < f0 where fO is the “cutoff” frequency.

WO = hfO

The number of photo electrons depends upon:
The nature of material
The frequency of incident radiation
The intensity of incident radiation
Potential difference b/w the electrons

Photon Theory of Light
According to the theory, light is an electromagnetic radiation with a
wavelength that is visible to the human eye.

A photon is an elementary particle that defines the light observed.
According to Einstein, there are three basic or fundamental
dimensions to be considered, when studying the Photon Theory of
Light.

1) Intensity: The property of intensity that the light displays is related
1) Intensity: The property of intensity that the light displays is related
to the subject's perception of the brightness of the light.
to the subject's perception of the brightness of the light.
2) Frequency: The property of frequency that is displayed and
2) Frequency: The property of frequency that is displayed and
observed
observed

is
is

actually
actually

the
the

color
color

of
of

the
the

light
light

perceived.
perceived.

3) Polarization: Contrary to the other two, the property of polarization
3) Polarization: Contrary to the other two, the property of polarization
of the light observed is only weakly perceptible, under
of the light observed is only weakly perceptible, under
ordinary circumstances.
ordinary circumstances.

According to the Albert Einstein's Photon Theory of Light, the
intensity of light shining on a metal determines the ability of the surface
to reflect and deflect the light.
It provides for observation the ability of a metal surface to receive and
throw out the light effectively and in an intensity that is observed to be
stronger than any other ordinary surface material.

Einstein suggested that, given the success of Planck’s theory, light
must be emitted in small energy packets:
.

These tiny packets, or particles, are called
photons.

The light is giving its energy to electrons in the atoms of the metal
The light is giving its energy to electrons in the atoms of the metal
and allowing them to move around, producing the current.
and allowing them to move around, producing the current.
However, not all colours of light affect metals in this way.
However, not all colours of light affect metals in this way.
No matter how bright aa red light you have, it will not produce aa
No matter how bright red light you have, it will not produce
current in aametal, but even aavery dim blue light will result in aacurrent
current in metal, but even very dim blue light will result in current
flowing.
flowing.
The problem was that these results can't be explained if light is
The problem was that these results can't be explained if light is
thought of as aawave.
thought of as wave.

Waves can have any amount of energy you want --big waves have aalot
Waves can have any amount of energy you want big waves have lot
of energy, small waves have very little.
of energy, small waves have very little.
And if light is aawave, then the brightness of the light affects the amount
And if light is wave, then the brightness of the light affects the amount
of energy -- the brighter the light, the bigger the wave, the more energy it
of energy the brighter the light, the bigger the wave, the more energy it
has.
has.
The different colours of light are defined by the amount of energy they
The different colours of light are defined by the amount of energy they
have.
have.
If all else is equal, blue light has more energy than red light with yellow
If all else is equal, blue light has more energy than red light with yellow
light somewhere in between.
light somewhere in between.
But this means that if light is aa wave, aa dim blue light would have the
But this means that if light is wave, dim blue light would have the
same amount of energy as aavery bright red light.
same amount of energy as very bright red light.

~~THE END~~

“Genius is eternal
patience”

- MIChELanGeLo

Chapter 3 photoelectric effect

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