1. AN INTRODUCTION TO…
MODERN PHYSICS

2. Let There be Light… They state (basically):
In 1873, James Clerk Maxwell summarized in 4 equations everything that was known about EM and EM waves
They state (basically):
Electric field lines start on a positive charge and end on a negative charge.
Magnetic field line form closed loops and have no beginning or end

3. Maxwell’s Thoughts cont…
3. An electric field must exist around a changing magnetic field. 4. A changing electric field generates a magnetic field.
These equations were thought to be the “be all and end all” of Physics. They fully explained everything known about electromagnetic waves. They were used to predict the speed of these waves.

5. Catastrophe!
The first problem for Maxwell’s eq’ns started with incandescence.
I’ve got a bad feeling about this…
James Maxwell
Gustav Kirchoff

6. Catastrophe!
Incandescence: heating matter until it emits electromagnetic radiation.
Kirchoff ( ) thought of a “blackbody radiator”
According to Maxwell’s eq’ns: PαT4 and fmax α T

7. Catastrophe! This relationship worked- sort of….
It worked until the frequency of light emitted was higher than visible light.
This was what was expected:

8. Catastrophe!

9. Catastrophe!
This is what occurred: p

10. Catastrophe!
This was called the UV catastrophe – the top minds of the day couldn’t figure out why this occurred.
Enter Max Planck- one of Kirchoff’s grad students

11. Planck to the rescue…
Planck came up with a formula to get the curve we saw earlier. It was an empirical model.
The theory behind this model was that ENERGY WAS QUANTIZED!
Energy was made up of discrete energy amounts

12. The Beginning of Quantum Physics
Planck found that the vibrations of the atoms in the radiator had certain energy levels.
He found that the energy levels were a constant multiplied by the frequency: E = hf
Finally a constant that’s not a k! That’s OK!

13. The Beginning of Quantum Physics
The energy of the oscillations were found to be: 0hf, 1hf, 2hf, 3hf, …
So, E = nhf, where n = 0, 1,2,3,…
‘h’ is called Planck’s constant and is
h = x 10 ^-34 Js
Planck’s model was purely mathematical and worked only if energy was quantized. The theory behind this was still a mystery.

14. Shedding light on Problem #2
Physicists’ understanding of EM was challenged with another experiment: The Photoelectric Effect
In the experiment, light was incident on a metal plate which was attached to a wire. The wire was connected to a galvanometer (a sensitive ammeter).

15. The Photoelectric Effect
A simulation of the Photoelectric effect

16. The Photoelectric Effect
When certain light shines on the metal, current passes through the ammeter. What is happening?
It must be that electrons are being ejecting from the metal. But why?

17. The Photoelectric Effect
Philipp Lenard also figured out that if you switch the battery poles, you can stop the electrons from hitting the collector plate. But again…why?

18. The Photoelectric Effect
To solve these questions, physicists called on probably the greatest physicist ever…

19. The Photoelectric Effect
Einstein developed the theory behind this and won the Nobel Prize for it.
To fully understand the significance of Einstein’s theory we must look at what physicists expected to happen and then we really did occur during the experiment.

20. The Photoelectric Effect
Experiment 1:Change the colour of light
Use applet and red to purple with cobalt to show that the effect does not occur with all light. (I = 50%), V = 0V
What we saw was that when CERTAIN light hitting a metal, electrons from the metal were ejected. From now on these electrons will be called PHOTOELECTRONS indicating they were emitted through the photoelectric effect.
Increase the intensity of the light in the applet.
Not all light caused the photoelectric effect to occur!

21. The Photoelectric Effect
Experiment 2:Change the intensity of light
Use applet and red increase the intensity to 100%. use cobalt to show that the effect does not occur
Use purple and increase the intensity to see more current
Return it to red and leave running
What we saw was that the intensity of the light DOES NOT affect whether or not the electrons are ejected.
If the photoelectric effect occurs, increasing the intensity will increase the current.
Increase the intensity of the light in the applet.

22. The Photoelectric Effect
Classical physics predicts that changing the intensity will increase the energy of the wave and thus the electrons should be emitted with higher intensity.
Remember: for a wave I = P/A = E/t/A so I α E.
More intense waves carry more energy.
Increase the intensity of the light in the applet.
But this doesn’t happen!

23. The Photoelectric Effect
Since light is a wave
resonance must be the factor
behind the emitted electrons. Light strikes an electron at the electron vibrates with the frequency of the light ray. As more and more light rays hit this electron, it will gain energy. (Think pushing someone on a swing.) This is possible since resonance can do this…
Example 2

24. The Photoelectric Effect
If resonance is the cause then maybe the red light needs more TIME to cause the electrons to be ejected. Let’s look back at the simulation.

25. The Photoelectric Effect
Still no current. The effect either occurs with that light or it never will. This is NOT what classical physics predicts. Experiment 3: Introducing a potential Use a neg voltage to show that the current drops but not to zero. Some of the electrons are stopped but not all. Increase the voltage to show the stopping potential
When a negative voltage is introduced, the current drops = some electrons don’t have the energy required to make it through the voltage to the collector plate. B) Increasing the intensity of the light has no effect on whether the effect occurs or not.

26. Experiment 3
We see that adding a negative voltage drops the current in the circuit. Why?
Some electrons are not able to pass through the potential we’ve created.
Board

27. Experiment 3
We see that adding a negative voltage drops the current in the circuit. Why?
Some electrons are not able to pass through the potential we’ve created.
Board
Potential

28. Experiment 3
What can we conclude about the marbles if they travelled different distances up the ramp?
They must have different amount of kinetic energy. So too must the photoelectrons since only some of them pass through the potential barrier (voltage). Show PhEt sim
However, if light is a wave then the intensity is related to the energy it has. So if we change the intensity, we can change the stopping potential. But this does not happen!

29. What we see now shows us that
1) that the photoelectric effect occurring is independent of the light’s intensity but is dependant on the light’s frequency.
2) the photoelectrons are emitted without delay
3) the photoelectrons have different energies and this energy is independent on the intensity of the light.
Classical Physics is…

30. The Photoelectric Effect
Einstein (who never worked in the lab!) studied these results and came up with the idea that light is a PARTICLE, not a wave.
He took Planck’s idea of energy being quantized and applied it to light. He called a packet of light energy a PHOTON. Each photon of light has a certain energy pertaining to its colour (its frequency).

31. The Photoelectric Effect
If the photon has the right energy, it can knock the electron off the metal.
Einstein also recalled Lenard’s contribution: reversing the polarity of the battery enables us to control and stop the current.
Einstein realized that what was happening was that these photoelectrons had different energies. That’s why when the polarity was reversed, the current decreases – some of the electrons are “sucked back”.

32. The Photoelectric Effect

33. The Photoelectric Effect
When all the electrons are sucked back, the voltage is called the stopping potential. It has enough potential to “use up” the kinetic energy of the most energetic electron.
Charge of photoelectron
Remember:
ping Potential
Electric Energy

34. The Photoelectric Effect
So the potential energy the Frisbee has here…
This height represents the “stopping potential” of the Frisbee
equals the kinetic energy it had here!

35. The Photoelectric Effect
Experiment 4: Finding the stopping potential:
λ = 494nm
I = 100%
V = -0.20V
Unclick Show electrons with max energy only.

36. The Photoelectric Effect
By measuring the voltage require to stop the photoelectrons from hitting the collector plate, we are calculating ….
We know now the MAXIMUM kinetic energy of a photoelectron for the experiment. Why do photoelectrons have different energies anyways?

37. The Work Function Let’s take a close look at the surface of a metal:
The first electron used some of the energy to move to the top of the surface. Then it used some to be ejected off the surface. Its kinetic energy was the left over energy. The second electron was already at the top of the surface so it just lost the energy required to be ejected: the work function! That photoelectron will have the max. kinetic energy.

38. The Results: Graphically
Work Function
Threshold frequency

39. The Equation of the Line
As we know the equation of a line is y =mx+b. This graph is no different:
KE(max) = hf - W
A few things to remember:
The energy of a photon = hf
The threshold frequency, f0, and the work function are related: W=hf0
The KE(max) is measured in eV. 1eV = 1.6 x 10^-19J
The KE(max) is calculated by using the stopping potential (V*q)

40. Examples
Calculate the energy of the green photon and the velocity of the photoelectron (blue incident light)

41. Photoelectric Effect Problems
1.Calculate the energy of a photon of blue light with a frequency of 6.67 x 1014 Hz. (State in eV) [2.76eV]
2.Calculate the energy of a photon of red light with a wavelength of 630 nm. [1.97eV]
3. Light of wavelength 600nm is directed at a metallic surface with a work function of 1.60 eV. Calculate
a. the maximum kinetic energy of the photoelectron. [Ek = 7.6 x 10-20]
b. the maximum speed of the photoelectron. [v = 4.1 x 105m/s]
c. the stopping potential of the metal. [v0 = 0.48 v]
4. Barium has a work function of 2.48 eV. What is the maximum kinetic energy of the ejected electron if the metal is illuminated by light of wavelength 450 nm? [0.28 eV]
5. When a 350nm light ray falls on a metal, the maximum kinetic energy of the photoelectron is 1.20eV. What is the work function of the metal? [2.3 eV]

42. Blackbody
a small hole in the side of a large box is an excellent absorber, since any radiation that goes through the hole bounces around inside, a lot getting absorbed on each bounce, and has little chance of ever getting out again. So, we can do this in reverse: have an oven with a tiny hole in the side, and presumably the radiation coming out the hole is as good a representation of a perfect emitter as we’re going to find.