1. Chapter 27- Atomic/Quantum Physics

2. The Sun http://soho.nascom.nasa.gov/
Why do we see the sun as yellow instead of green or blue or pink?

3. Blackbody Spectrum

4. Photon Theory of Light
Light is transmitted as tiny particles called photons
The amount of energy in a photon depends on its frequency
h= Planck’s Constant= x Js
f= frequency of light
c= speed of light
λ= wavelength
p= momentum

5. The Photoelectric Effect
When light shines on a metal surface, electrons are emitted from the surface

6. Photocells (p.829)
When the photocell is in the dark, the ammeter reads 0 (no current)
When light with a high enough frequency shines on the current flows in the circuit

7. Photocells
KEmax of the emitted electrons can be found by reversing the voltage and making the C electrode negative
The electrons are repelled by C, but the fastest electrons will still make it across
There is a minimum voltage, Vo, called the stopping voltage. No current will flow if the voltage is less than the stopping voltage
Kemax = e Vo

8. Photoelectric Effect Einstein’s Theory Predicts:
Increasing the intensity of the light does not make the electrons go faster.
This is because although more photons are striking the surface, they have same energy
Increasing the frequency of the light beam increases the energy of the photons which changes the maximum KE of the ejected electrons

9. Photoelectric Effect
The work function, Wo, is the minimum amount of energy necessary to get an electron off the surface of the metal
fo is the “cutoff” frequency. If the light beam’s frequency is below that, then no electrons will be emitted

10. Compton Effect A.H. Compton scattered xrays from various materials
Found out that the scattered light had a lower frequency than incident light
Since frequency decreases, wavelength increases
Used conservation of momentum to determine that the photon transfers some of its energy to the electron

11. de Broglie Wavelength
Light sometimes behaves like a wave and sometimes like a particle
Louis de Broglie came up with the idea that particles might also have wave properties
De Broglie wavelength of a particle

12. de Broglie Wavelength The wavelength of large objects is very small
For a 0.20 kg ball travelling at 15 m/s

13. de Broglie Wavelength
Determine the wavelength of an electron that has been accelerated through a potential difference of 100 V

14. Davisson-Germer Experiment
The spacing of atoms in a crystals is on the order of m, so one could be used as a diffraction grating
In 1927, Davisson and Germer scattered electrons from the surface of a metal crystal. The wavelength they got matched the predicted de Broglie wavelength

15. Bohr Model of the Atom
Electrons orbit the nucleus in circular orbits called stationary states
When an electron jumps from one state to another, light is either absorbed or emitted
The energy required to go between states is a fixed amount

16. Bohr Model of the atom
If an electron jumps from a higher state to a lower state, it emits a single photon of light

17. Energy Level Diagram (p. 847)
n= 1is ground state, n=2,3,4.. Are excited states
To completely free an electron in the ground state, you’d need to put in 13.6 eV of energy (ionization energy of Hydrogen)

18. Energy Level Diagram How much energy to go from ground to n=2?
How much energy to go from n=2 to n=4?

19. Energy Level Diagrams
What are the possible transitions for an electron in excited state n=3?
31
32
21

20. Emission Spectra
A material’s emission spectrum show the wavelengths of the photons emitted when electrons jump to lower energy states

21. Absorption Spectra
The absorption spectrum of a material shows that gases can absorb light at the same frequencies at which they emit

22. Absorption/Emission Spectra