LIGHT and MATTER

1. LIGHT and MATTER Chapters 12 and 13

2. THE DOUBLE-SLIT EXPERIMENT Originally performed by Young (1801) to demonstrate the wave-nature of light. Has now been done with electrons, neutrons, He atoms among others. Alternative method of detection: scan a detector across the plane and record number of arrivals at each point. y d θ Incoming coherent beam of particles (or light) Detecting screen D For particles we expect two peaks, for waves an interference pattern

3. PHOTOELECTRIC EFFECT Classical expectations 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 J.J. Thomson Hertz 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). Light, frequency ν Vacuum chamber Metal plate Collecting plate I Ammeter Potentiostat

4. PHOTOELECTRIC EFFECT But what actually happened? http://phet.colorado.edu/en/simulation/photoelectric Energy of emitted electrons did not depend on the intensity of the light Each material had a unique cut-off frequency The graph of stopping voltage versus frequency was a straight line of slope = “h”.

5. Collimator (selects angle) X-ray source Crystal (selects wavelength) θ Target COMPTON SCATTERING 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).

6. De BROGLIE WAVELENGTH If light can exhibit particle like behaviour, can matter exhibit wavelike behaviour? http://www.youtube.com/watch?v=uPPyYhHOPb0

7. 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

8. 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?

9. Photoelectric Effect Number 2 This minimum voltage which causes all electrons to turn back is called the stopping voltage. Number 1 Higher intensity light produced greater values of the maximum photocurrent Number 3 Brighter light did not increase the kinetic energy of the electrons emitted from the cathode the maximum photocurrent was directly proportional to the light intensity

10. 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.

11. The Electron Volt

12. Photoelectric Effect

13. Photoelectric Effect