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 =