light and matter chapters 11 & 12. originally performed by young (1801) to demonstrate the...
DESCRIPTION
PHOTOELECTRIC EFFECT When UV light is shone on a metal plate in a vacuum, it emits charged particles (Hertz 1887), which were later shown to be electrons by J.J. Thomson (1899). Electric field E of light exerts force F=-eE on electrons. As intensity of light increases, force increases, so KE of ejected electrons should increase. Electrons should be emitted whatever the frequency ν of the light, so long as E is sufficiently large For very low intensities, expect a time lag between light exposure and emission, while electrons absorb enough energy to escape from material Classical expectations Hertz J.J. Thomson I Vacuum chamber Metal plate Collecting plate Ammeter Potentiostat Light, frequency νTRANSCRIPT
LIGHT and MATTER
Chapters 11 & 12
sind
Originally performed by Young (1801) to demonstrate the wave-nature of light. Has now been done with electrons, neutrons, He atoms among others.
D
θd
Detecting screen
Incoming coherent beam of particles (or light)
y
Alternative method of detection: scan a detector across the plane and record number of arrivals at each point.
THE DOUBLE-SLIT EXPERIMENT
For particles we expect two peaks, for waves an interference pattern
PHOTOELECTRIC EFFECTWhen UV light is shone on a metal plate in a vacuum, it emits charged particles (Hertz 1887), which were later shown to be electrons by J.J. Thomson (1899).
Electric field E of light exerts force F=-eE on electrons. As intensity of light increases, force increases, so KE of ejected electrons should increase.Electrons should be emitted whatever the frequency ν of the light, so long as E is sufficiently largeFor very low intensities, expect a time lag between light exposure and emission, while electrons absorb enough energy to escape from material
Classical expectations
Hertz J.J. Thomson
I
Vacuum chamber
Metal plate
Collecting plate
Ammeter
Potentiostat
Light, frequency ν
COMPTON SCATTERING
X-ray source
Target
Crystal (selects wavelength)
Collimator (selects angle)
θ
Compton (1923) measured intensity of scattered X-rays from solid target, as function of wavelength for different angles. He won the 1927 Nobel prize.
Result: peak in scattered radiation shifts to longer wavelength than source. Amount depends on θ (but not on the target material).
But what actually happened?
http://phet.colorado.edu/en/simulation/photoelectric
Energy of emitted electrons did not depend on the intensity of the lightEach material had a unique cut-off frequencyThe graph of stopping voltage versus frequency was a straight line of slope = “h”.
PHOTOELECTRIC EFFECT
Free electrons must escape
Gradient = h
Work FunctionW = hfo
where h = 6.63 X 10 -34 js-1 and fo is the threshold frequency
Ek (max) = ½ mvmax2 = hf –W
= max KE of released electronsWhere hf = incident photon energyW = work function
Momentum?
Longer wavelength?
Momentum of a photon
De Broglie and matter waves
Electron and X-rays
Cricket balls
If light can exhibit particle like behaviour, can matter exhibit wavelike behaviour?
http://www.youtube.com/watch?v=uPPyYhHOPb0
De BROGLIE WAVELENGTH
What is Light? What is Matter?
WAVE-PARTICLE DUALITY
In 1924 Einstein wrote:- “ There are therefore now two theories of light, both indispensable, and … without any logical connection.”
Evidence for wave-nature of light• Diffraction and interference
Evidence for particle-nature of light• Photoelectric effect• Compton effect
SAC• Can you sketch a diagram illustrating the double slit interference
experiment?• Can you explain why light passing through two narrow slits produces a
pattern?• Do you know why double slit interference supports the wave theory of
light?• Can you describe the photoelectric effect experiment ie. how it was
conducted, the apparatus used and what results were obtained?• Do you know why the wave theory could not explain the photoelectric
effect?• Can you sketch a diagram of an electron diffraction experiment?• Do you know how this experiment showed that particles can behave
like waves?
Photoelectric EffectNumber 1Higher intensity light produced greater values of the maximum photocurrent
the maximum photocurrent was directly proportional to the light intensity
Number 2This minimum voltage which causes all electrons to turn back is called the stopping voltage.
Number 3Brighter light did not increase the kinetic energy of the electrons emitted from the cathode
Photoelectric Effect
4.The stopping voltage depended on both the frequency of the light and on the material of the electrode. In fact, for each material there was a minimum frequency required for electrons to be ejected. Below this cut-off frequency no electrons were ever ejected, no matter how intense the light or how long the electrode was exposed to the light. Above this frequency a photocurrent could always be detected. The photocurrent could be detected as quickly as 10-9 s after turning on the light source. This time interval was independent of the brightness of the light source.
The Electron Volt
Photoelectric Effect
Photoelectric Effect
Atomic Viewpoint
• Were deflected by magnetic fields• Could push a paddle wheel and carried momentum• Could pass through metals without damaging them• Travelled more slowly than light• Were negatively charged• Carried energy• Travelled in straight lines
Cathode Rays
Cathode Rays
Emission Spectra
Emission Spectra
Emission vs Absorption Spectra
Emission Spectrum
Absorption Spectrum
Equations
• Ek = ½ mv2
= qeV
• p = = So ʎ =