Chapter 3 photoelectric effect

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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.


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.


PHOTOEMISSIVEPHOTOEMISSIVE

A material that can exhibit the

photoelectric effect

A material that can exhibit the

photoelectric effect

PHOTOELECTRONSPHOTOELECTRONS

The ejected electronsThe 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 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.


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.


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

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


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.


V

I

Vo

Saturation currenti

At intensity I

At intensity II

At intensity III


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

V(Vo )2 (Vo )1

Constant intensity

Saturation current

i


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..


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, 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.


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


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.


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.

No electrons are emitted if f < f0 where fO is the “cutoff” frequency.

Kmax = hf - WO

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


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

http://www.blurtit.com/q968609.html

http://www.blurtit.com/q968609.html


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.


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.


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.


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

-

Chapter 3 photoelectric effect

Technology

Transcript of Chapter 3 photoelectric effect