chapter 3 photoelectric effect
TRANSCRIPT
ChaPtEChaPtER 3 :R 3 :
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iC EffeCtiC EffeCt
SCOPE OF STUDYSCOPE 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 PHOTOELECTRIC EFFECTEFFECTDEFINITION DEFINITION
It's been determined experimentally that
when light shines on a metal surface, the
surface emits electrons
It's been determined experimentally that
when light shines on a metal surface, the
surface emits electrons
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
Example : You can start a current in a circuit just by shining a light on
a metal plate.
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
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.
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
Example :
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
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.
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
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.
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
PHOTOEMISSIVEPHOTOEMISSIVE
A material that can exhibit the
photoelectric effect
A material that can exhibit the
photoelectric effect
PHOTOELECTRONSPHOTOELECTRONS
The ejected electronsThe ejected electrons
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
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.
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
Wave Theory PredictionsWave Theory Predictions
If light is a wave, theory predicts:
1. If the light intensity increase, the number of electrons
ejected and their maximum kinetic energy increase.
Because the higher intensity means a greater electric field
amplitude and the greater electric field should eject
electrons with higher speed.
2. The frequency of light not affect the kinetic energy of the
ejected electrons.
If light is a wave, theory predicts:
1. If the light intensity increase, the number of electrons
ejected and their maximum kinetic energy increase.
Because the higher intensity means a greater electric field
amplitude and the greater electric field should eject
electrons with higher speed.
2. The frequency of light not affect the kinetic energy of the
ejected electrons.
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
Photon Theory PredictionsPhoton Theory Predictions
If light is particles, theory predicts:
• Increasing intensity increases number of electrons but not
energy.
• Above a minimum energy required to break atomic bond,
kinetic energy will increase linearly with frequency.
• There is a cutoff frequency below which no electrons will be
emitted, regardless of intensity.
If light is particles, theory predicts:
• Increasing intensity increases number of electrons but not
energy.
• Above a minimum energy required to break atomic bond,
kinetic energy will increase linearly with frequency.
• There is a cutoff frequency below which no electrons will be
emitted, regardless of intensity.
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
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.
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
*To determine the maximum kinetic energy, K max
from VO, use the conservation of energy :
Loss of kinetic energy = Gain in potential energy
K K maxmax = e V = e VOO
*To determine the maximum kinetic energy, K max
from VO, use the conservation of energy :
Loss of kinetic energy = Gain in potential energy
K K maxmax = e V = e VOO
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
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.
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
V
I
Vo
Saturation currenti
At intensity I
At intensity II
At intensity III
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
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.
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
I
V(Vo )2 (Vo )1
Constant intensity
Saturation current
i
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
OTHER FUNDAMENTAL LAWS OF PHOTO ELECTRIC EMISSION
OTHER FUNDAMENTAL LAWS OF PHOTO 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..
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
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.
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
Work Function, WOWork Function, WO
Minimum amount of energy which is necessary to
start photo electric emission
Minimum amount of energy which is necessary to
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.
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
All the photon energy is transferred to the electron and the photon
ceases to exist.
Electrons are held in the metal by attractive forces, some minimum
energy, WO is required just to get an electron out through the surface.
If hf < Wo , the photons will not have enough energy to eject any
electrons at all.
If hf > Wo , electrons will be ejected and energy will be conserved
in the process. This will come out equation : hf = K + W
If the least bound electrons, hf = Kmax + Wo
All the photon energy is transferred to the electron and the photon
ceases to exist.
Electrons are held in the metal by attractive forces, some minimum
energy, WO is required just to get an electron out through the surface.
If hf < Wo , the photons will not have enough energy to eject any
electrons at all.
If hf > Wo , electrons will be ejected and energy will be conserved
in the process. This will come out equation : hf = K + W
If the least bound electrons, hf = Kmax + Wo
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
THRESHOLD FREQUENCY, foTHRESHOLD FREQUENCY, fo
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.
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
The particle theory assumes that an electron absorbs a single photon.
Plotting the kinetic energy vs. frequency:
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
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.
No electrons are emitted if f < f0 where fO is the “cutoff” frequency.
Kmax = hf - WO
WO = hfO
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
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
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
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.
Photon Theory of Light
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
1) Intensity: The property of intensity that the light displays is related
to the subject's perception of the brightness of the light.
2) Frequency: The property of frequency that is displayed and
observed is actually the color of the light perceived.
3) Polarization: Contrary to the other two, the property of polarization
of the light observed is only weakly perceptible, under
ordinary circumstances.
1) Intensity: The property of intensity that the light displays is related
to the subject's perception of the brightness of the light.
2) Frequency: The property of frequency that is displayed and
observed is actually the color of the light perceived.
3) Polarization: Contrary to the other two, the property of polarization
of the light observed is only weakly perceptible, under
ordinary circumstances.
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
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.
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
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.
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
Failure of wave optics in explaining the photoelectric effect Failure of wave optics in explaining the photoelectric effect
The light is giving its energy to electrons in the atoms of the metal
and allowing them to move around, producing the current.
However, not all colours of light affect metals in this way.
No matter how bright a red light you have, it will not produce a
current in a metal, but even a very dim blue light will result in a current
flowing.
The problem was that these results can't be explained if light is
thought of as a wave.
The light is giving its energy to electrons in the atoms of the metal
and allowing them to move around, producing the current.
However, not all colours of light affect metals in this way.
No matter how bright a red light you have, it will not produce a
current in a metal, but even a very dim blue light will result in a current
flowing.
The problem was that these results can't be explained if light is
thought of as a wave.
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
Waves can have any amount of energy you want - big waves have a lot
of energy, small waves have very little.
And if light is a wave, then the brightness of the light affects the amount
of energy - the brighter the light, the bigger the wave, the more energy it
has.
The different colours of light are defined by the amount of energy they
have.
If all else is equal, blue light has more energy than red light with yellow
light somewhere in between.
But this means that if light is a wave, a dim blue light would have the
same amount of energy as a very bright red light.
Waves can have any amount of energy you want - big waves have a lot
of energy, small waves have very little.
And if light is a wave, then the brightness of the light affects the amount
of energy - the brighter the light, the bigger the wave, the more energy it
has.
The different colours of light are defined by the amount of energy they
have.
If all else is equal, blue light has more energy than red light with yellow
light somewhere in between.
But this means that if light is a wave, a dim blue light would have the
same amount of energy as a very bright red light.
~~THE END~~~~THE END~~
“Genius is eternal
patience”
- MIChELanGeLo
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