© John Parkinson 1 MAX PLANCK PHOTOELECTRIC EFFECT.
-
Upload
jonathan-schmidt -
Category
Documents
-
view
247 -
download
2
Transcript of © John Parkinson 1 MAX PLANCK PHOTOELECTRIC EFFECT.
©John Parkinson
1
MAX PLANCK
PHOTOELECTRIC PHOTOELECTRIC EFFECTEFFECT
©John Parkinson
2
THE PHOTOELECTRIC EFFECT
THIS IS THE EMISSION OF ELECTRONS FROM MATTER WHEN MATTER IS ILLUMINATED BY CERTAIN TYPES OF ELECTROMAGNETIC RADIATION.
THE EFFECT OCCURS WHEN METALS ARE ILLUMINATED BY UV LIGHT AND CAN OCCUR WITH THE
ALKALI METALS FOR VISIBLE LIGHT.
IT WAS FIRST OBSERVED BY HEINRICH HERTZ IN 1887
©John Parkinson
3
Radiation
mA
Anode +ve Cathode -ve
electrons
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
vacuum
©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.
mA
A C
Radiation
electronselectrons
The p.d. across the tube measures the maximum kinetic energy of the ejected
electrons in electron volts.
V
©John Parkinson
5
At the end of the nineteenth century, Classical Electromagnetic Wave Theory thought of light
waves as being like water waves.
The wave’s Intensity or energy was directly proportional to the square of the Amplitude, A.
A
©John Parkinson
6
Potassium metal undergoes photoemission with blue and green light, but not with red light.
potassium metal
Emission!Emission!
Nothing!!
Blue light
Green light
Red light
©John Parkinson
7
THE CLASSICAL THEORY SUGGESTS TRYING MORE INTENSE LIGHT
potassium metal
Nothing!!Nothing!!
©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 10-34 Js]
But velocity of light = frequency times wavelength fc
Substituting c
f into E = hf
hc
EENERGYPHOTON
©John Parkinson
10
hc
EENERGYPHOTON
the visible spectrum
λ
frequency
violet light light 400 nm
red light light 700 nm
uv light < 400 nm
Blue photon Red photon
Which photon has the most energy ?????
BLUE !!!
©John Parkinson
11
Quantum Theory of the Photoelectric Effect
Because of the interaction of this electron with other atoms, it requires a certain minimum energy to escape from the
surface.
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 eithershare its excess energy with other electrons and the ion
lattice or it can use the excess energy to fly out of the metal.
The minimum energy required to escape depends on the metal and is called the work function, Φ.
©John Parkinson
12
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 = hf - Φ For every metal there is a threshold frequency, f0, where hf0 = Φ ,that gives the photon enough energy to produce photoemission.
It follows that the photo electric current is proportional to the intensity of the radiation provided the frequency of radiation is
above threshold frequency.
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
EK = photon energy – the work function.
©John Parkinson
13
Maximum EK emitted electrons / J
Frequency f / Hz
metal A
Work function, Φ
Threshold frequency f0
metal B
EK = hf - Φ
Gradient of each graph = Planck’s constant, h.
©John Parkinson
14
f / Hz 1014
0 5 10 15
Max Ek / eV
1
2
Potassium Magnesium Aluminium
©John Parkinson
15
Summary
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.
For incident radiation of a given frequency, the number of electrons emitted per second is proportional to the intensity of the radiation.
Electron emission takes place immediately after the light shines on the metal with no detectable time delay .