1. MAX PLANCK
PHOTOELECTRIC EFFECT
John Parkinson

2. THE PHOTOELECTRIC EFFECT
THIS IS THE EMISSION OF ELECTRONS FROM MATTER WHEN MATTER IS ILLUMINATED BY CERTAIN TYPES OF ELECTROMAGNETIC RADIATION.
IT WAS FIRST OBSERVED BY HEINRICH HERTZ IN 1887
THE EFFECT OCCURS WHEN METALS ARE ILLUMINATED BY UV LIGHT AND CAN OCCUR WITH THE ALKALI METALS FOR VISIBLE LIGHT.
John Parkinson

3. The electromagnetic radiation releases electrons from the metal cathode. These electrons are attracted to the anode and complete a circuit allowing a current to flow
Radiation mA
Anode +ve
vacuum
Cathode -ve
electrons
John Parkinson

4. If the polarity is reversed, the pd across the tube can be increased until even the most energetic electrons fail to cross the tube to A. The milliammeter then reads zero.
Radiation mA A C
electrons
electrons V
The p.d. across the tube measures the maximum kinetic energy of the ejected electrons in electron volts.
John Parkinson

5. At the end of the nineteenth century, Classical Electromagnetic Wave Theory thought of light waves as being like water waves. A
The wave’s Intensity or energy was directly proportional to the square of the Amplitude, A.
John Parkinson

6. Potassium metal undergoes photoemission with blue and green light, but not with red light.
Blue light
Emission!
Emission! e
Green light
Red light
Nothing!!
potassium metal
John Parkinson

7. THE CLASSICAL THEORY SUGGESTS TRYING MORE INTENSE LIGHT
Nothing!!
Nothing!!
potassium metal
John Parkinson

8. The Classical Theory must be wrong!!!!!
John Parkinson

9. Quantum Theory of the Photoelectric Effect
In 1905 Einstein developed Planck’s idea, that energy was quantised in quanta or photons, in order to explain the photoelectric effect.
Electromagnetic radiation is emitted in bursts of energy – photons. The energy of a photon is given by E = hf, where f is the frequency of the radiation and h is Planck’s constant. [h = 6.6 x Js]
But velocity of light = frequency times wavelength
Substituting
into E = hf
John Parkinson

10. BLUE !!! λ frequency the visible spectrum violet light light 400 nm
red light
light 700 nm
uv light < 400 nm λ
Blue photon
Red photon
Which photon has the most energy ?????
BLUE !!!
John Parkinson

11. The electron can then either
Quantum Theory of the Photoelectric Effect
The photons are sufficiently localized, so that the whole quantum of energy [ hf ] can be absorbed by a single electron at one time.
The electron can then either
share its excess energy with other electrons and the ion lattice or it can use the excess energy to fly out of the metal.
Because of the interaction of this electron with other atoms, it requires a certain minimum energy to escape from the surface.
The minimum energy required to escape depends on the metal and is called the work function, Φ.
John Parkinson

12. The maximum kinetic energy the released electron can have is given by:
For electron emission, the photon's energy has to be greater than the work function .
The maximum kinetic energy the released electron can have is given by:
EK = photon energy – the work function.
EK = hf - Φ
For every metal there is a threshold frequency, f0. The photon energy h f0 is that required to just release the most energetic electrons from the metal.
The number of photoelectrons emerging from the metal surface per unit time is proportional to the number of photons striking the surface that in turn depends on the intensity of the incident radiation
It follows that the photo electric current is proportional to the intensity of the radiation provided the frequency of radiation is above threshold frequency.
John Parkinson

13. Maximum EK emitted electrons / J
metal A
Maximum EK emitted electrons / J
metal B
Threshold frequency f0
Frequency f / Hz
EK = hf - Φ
Work function, Φ
Gradient of each graph = Planck’s constant, h.
John Parkinson

14. Potassium Magnesium Aluminium Max Ek / eV 2 1 0 5 10 15 f / Hz 1014 ©
Max Ek / eV 1 2
Potassium
Magnesium
Aluminium
John Parkinson

15. Summary
For incident radiation of a given frequency, the number of electrons emitted per second is proportional to the intensity of the radiation.
For any metal there is a minimum threshold frequency, f0, of the incident radiation, below which no emission of electrons takes place, no matter what the intensity of the incident radiation is or for how long it falls on the surface.
Electrons emerge with a range of velocities from zero up to a maximum. The maximum kinetic energy, Ek, is found to depend linearly on the frequency of the radiation and to be independent of its intensity.
Electron emission takes place immediately after the light shines on the metal with no detectable time delay .
John Parkinson