1. CH 221 Computer Simulation Activity
Photoelectric Effect
Physics
Electromagnetic Spectrum
n = c/l

2. Practical applications of the photoelectric effect
solar cells
smoke detectors
security systems

3. Many metals emit electrons when a light shines on their surface
Many metals emit electrons when a light shines on their surface. The emitted electrons are called photoelectrons.

4. Metal’s electrons near the surface
Photoelectric Effect
Light
Electrons
Metal’s electrons near the surface

5. What to look for during the experiment if light has wave behavior:
Photoelectric effect: Many metals emit electrons when a light shines on their surface.
What to look for during the experiment if light has wave behavior:
1. when the wavelength of light is made shorter, more electrons should be ejected. Why?
2. when the light wave’s intensity made brighter, more electrons should be ejected. Why?
3. when the light is dim, there should be a lag time in the ejection of electrons. Why?

6. Photoelectric Effect
In order to eject an electron, the force of attraction (work function or binding energy) between the protons and the electron must be overcome: Coulomb’s Law.

7. Coulomb’s Force Law
A law of physics describing the electrostatic interaction between charged particles.
Force of attraction between a proton and an electron.

8. Two unlike charges are separated by a distance, which set of charges experiences a greater force of attraction? A. B.

9. Two unlike charges are separated by a distance, which set of charges experiences a greater force of attraction? A. B.

10. The Photoelectric Effect
Classic wave theory attributed this effect to all of the energy of light E = hv being transferred to an outer electron in the metal.
According to this theory, if the wavelength of light hitting the metal is shorter, or the light wave’s intensity made brighter, more electrons should be ejected.
the energy of a wave is directly proportional to its amplitude and its frequency. The more energy light has, the more energy it should have to knock off electrons.
Wave theory also predicts if a dim light were used there would be a lag time before electrons were emitted. Dim light as waves were predicted to eject electrons if enough time was given for the electrons to absorb enough energy to break away from the force of attraction of the protons in the nucleus.

11. E = nhν
An atom has only certain quantities of energy… the energy is quantized.
n = 5
n = 4
n = 3
ΔE = Δnhν
n = 2
Note that because h is so small, delta E is also very small. Scientists at the time, including Planck, were uncomfortable with this theory, but it worked. Planck received Nobel prize in 1918 for his part in changing the way scientists view the world.
Because h is so small, this is only relevant for things on the atomic scale.
n = 1
The smallest energy change is when Δn = 1
ΔE = hν
a quantum of energy

12. Electrons exist in discrete energy levels

13. Classical View Compare to Quantum Mechanical View
Any energy is possible
Quantum Mechanical View
Only a few energy states are allowed

14. Energy and Frequency
A solid object emits visible light when it is heated to about 1000 K and above. This is called blackbody radiation.
Smoldering coal
Lightbulb filament
Electric heating element
Classical physics tells us that atoms in a solid vibrate around a fixed point. As the temperature increases, the vibration becomes more vigorous. Black-body radiation is the release of some of this vibrational energy as EMR. Classical physics did not have an explanation for this that worked at all frequencies and temperatures.
The color (and the intensity) of the light depend only on the temperature and not the identity of the substance.
What is the source of this light and what is the relationship between the frequency of the radiation (i.e., the color of the light) and the temperature of the body?

15. Historically, light was thought of as a stream of particles until Young’s experiments proved light has wave-like properties.
Planck was working with the wave notion of light when he related the energy of blackbody radiation to the frequency of emitted light. E = hv
Einstein began to consider the particle viewpoint again when trying to explain the photoelectric effect.
Max Planck and Albert Einstein
Berlin, June 1929

17. Photoelectric Effect Experiment Work the Questions of the Activity Worksheet posted on the Canvas web site
Download and work the computer simulation

18. Also is should take time for the surface e-s to be ejected.
Photoelectric effect experiment apparatus.
Test metal
Electrons
if light follows classical wave theory, we predict that any incoming light will put energy into the surface e-s
Also is should take time for the surface e-s to be ejected.
When the light on longer, more e’s should be ejected
Any color light (wavelength) should eject e-s, only an increase in the intensity of light should increase the KE of the e-s.

19. Keep the intensity of light constant
e’s
Select sodium metal
First experiment
Keep the intensity of light constant
Start with IR or Red light, increase the frequency of light until electrons are ejected.
Note the threshold frequency (and wavelength)
Observe what happens, write down what happens

20. Select two different metals
e’s
First experiment Part 2
Select two different metals
Design an experiment to determine the threshold frequency of each metal.
What variables will you keep constant?
Observe what happens, write down what happens

21. Select a frequency of light below the threshold frequency.
Select sodium metal
Second experiment
Select a frequency of light below the threshold frequency.
Increase the intensity of light
Select a frequency of light above the threshold frequency.
Make note of the speed and number of ejected electrons.
Observe what happens, write down what happens

22. Choose a frequency above the threshold frequency,. I
e’s
HIGH intensity
LOW intensity
Choose a frequency above the threshold frequency,.
Keep the frequency and wavelength of the light the same,
vary the intensity
Observation: When we dim the light,
fewer electrons are ejected from the metal.

23. Work with the two other different metals from Experiment 2
e’s
Third experiment
Work with the two other different metals from Experiment 2
- Determine the threshold frequency of each metal
Observe what happens, write down what happens

24. Select one of your metals. I
e’s
look at sim for few different
colors, small forward V
Select one of your metals.
Predict what happens to the initial KE of the electrons as the frequency of light changes? (Light intensity is constant)
Frequency of light
Initial KE
Predict shape of the graph

25. Work with the two other different metals from Experiment 2
e’s
Third experiment
Work with the two other different metals from Experiment 2
- Determine the threshold frequency of each metal
Determine if there is a relationship between the # of e-s ejected and the intensity of light
Determine if there is a relationship between the KE of the ejected electrons and the frequency of light.
Observe what happens, write down what happens

26. What is the relationship between KE and frequency?
Initial KE
Frequency
Initial KE A B
What is the relationship between KE and frequency?
Frequency
Initial KE
Frequency
Initial KE C D
E. something different

27. what happens if change metal? GOTO the simulation do the experiment
Correct answer is D.
do sim showing graph
There is a minimum frequency below which the light cannot kick out electrons…
even if wait a long time
As the frequency of light increases (shorter l!), the KE of electrons being popped off increases.
(it is a linear relationship)
Initial KE
what happens if change metal?
GOTO the simulation
do the experiment
0 Frequency of light

28. Threshold Frequency (Energy)

29. Each Metal Has a Different Threshold Frequency (Energy)

30. Which Metal Requires the Greatest (Minimum) Energy to Begin to Eject Electrons? What does this mean with respect to high tightly electrons are bound to an atom?

31. Metal’s electrons near the surface
Photoelectric Effect
Photo-
Light
Electrons
Metal’s electrons near the surface
The theory of the photoelectric effect must explain the experimental observations of the emission of electrons from an illuminated metal surface. For a given metal, there exists a certain minimum frequency of incident radiation below which no photoelectrons are emitted.

32. Summary of Photoelectric experiment results.
(play with sim to check and thoroughly understand)
1. # of e-s ejected linearly proportional to intensity of light.
2. # of e-s ejected appears with no delay.
3. Electrons only emitted if frequency of light exceeds
a threshold. (same as “if wavelength short enough”).
4. Maximum energy that electrons come off with
increases linearly with frequency (=1/wavelength).
5. Threshold frequency depends on the type of metal - - which is an indication of how tightly the surface e-s are held.
how do these compare with classical wave predictions?

34. Classical wave predictions vs. experimental observations
Increase intensity, increase # of electrons.
experiment matches
Wavelength of light does not matter, only intensity.
experiment shows strong dependence on wavelength
Takes time to heat up ⇒ # of e-s coming off start low and should increase with time (energy added).
experiment: with the threshold frequency electrons come off the surface immediately, no time delay to heat up

35. (Light energy must be getting concentrated/focused somehow)
Summary
If light can kick out electrons, then even smallest intensities of that light will continue to kick out electrons. KE of electrons does not depend on the intensity of the light of a specific wave length.
(Light energy must be getting concentrated/focused somehow)
2. At lower frequencies, initial KE of the ejected e-s is low. KE of the ejected e-s changes linearly with the frequency of the incoming light. (This concentrated energy is linearly related to frequency)
3. There is a minimum frequency below which light won’t kick out electrons.
(Need a certain amount of energy to free electron from metal)
(Einstein) Need “photon” picture of light to explain observations:
- Light comes in chunks (“particle-like”) of energy (“photon”)
- a photon interacts only with single electron
- Photon energy depends on frequency of light, …
for lower frequencies, photon energy not enough to free an electron

36. Einstein’s Model of Light: Photon Torpedoes
Light can be represented as separate, discontinuous quanta called photons.
Light energy comes in packets.
Each photon has an energy of E = hv
Light interacts with matter as a stream of particle-like photons.
Light travels as a wave.
Light is complex – it has a wave-particle duality behavior.

37. Photoelectric Effect Three Three Energy Out Energy In Metal Surface
Energy In > Energy Out What absorbed the missing energy?
Three
Three
Energy Out
Energy In
Metal Surface

38. Photoelectric Effect Experiment: Sodium metal has a threshold wavelength of 232 nm. Calculate the frequency and the energy of the incoming light.

39. Photoelectric Effect Experiment: Sodium metal has a threshold wavelength of 232 nm. Calculate the frequency and the energy of the incoming light.

40. or, Eincoming photon = Φ + KEejected photoelectron = hν
Einstein proposed that light consists of particles, or quanta, of energy called photons.
Energy of one photon = hν Energy of n photons = E = nhν Photons hit electrons on the surface of a metal (like billiard balls).
Photon energy is transferred completely to the electron.
The electron must spend some energy breaking away. This is the “binding energy” or “work function”, Φ.
Excess energy remains in the emitted electron as kinetic energy.
or, Eincoming photon = Φ + KEejected photoelectron = hν
Light below a minimum “threshold frequency” will not cause the photoelectric effect.

41. Metal’s electrons near the surface
Photoelectric Effect
Photo-
Energy In = hv
Light
Electrons
Energy Out
= Kinetic Energy
Metal’s electrons near the surface
Binding Energy
Photoelectron Energy = Light Energy In – Binding Energy
KEphotoelectron = hν – Φ

42. Photoelectric Effect: Calculate Binding Energy
If Energy In = 25 = E = hv
Light
Electrons
If Energy Out
KE = 5
Binding Energy
Photoelectron Energy = Light Energy In – Binding Energy
5 = 25 – Φ Φ = 25 – 5 = 20

43. Photoelectric Effect Photo- Electrons
Molybdenum metal must absorb radiation with a minimum frequency of 1.09 x 1015 cyc/sec before it can eject an electron from its surface. Calculate the energy (units of J) to eject an electron.
Energy In = hv h v = h x 1.09 x 1015 cyc/sec
Photo-
Electrons
Binding Energy = minimum energy needed to eject
an electron from the metal surface

44. l = c/v Photoelectric Effect Photo- Electrons
Molybdenum metal must absorb radiation with a minimum frequency of 1.09 x 1015 cyc/sec before it can eject an electron from its surface. What is the wavelength, l , of this radiation? What type of radiation is it?
Energy In = hv v = 1.09 x 1015 cyc/sec
Photo-
l = c/v
Electrons
Binding Energy

45. v = c/l Photoelectric Effect Photo- Electrons ? = hv – Φ
If Molybdenum is irradiated with EMR of wavelength nm, what is the maximum possible kinetic energy of the emitted photoelectrons?
Energy In = hv l = nm
Photo-
v = c/l
Electrons
If Energy Out
KE = ?
Binding Energy
Photoelectron Energy = Light Energy In – Binding Energy
? = hv – Φ

47. Photoelectric effect experiment: Apply Conservation of Energy
Energy in = Energy out
Energy of photon = energy needed to kick + Initial KE of electron electron out of metal as exits metal
Loosely stuck electron, takes least energy to kick out
work function () = energy needed to kick
highest electron out of metal
Electron Potential
Energy
Outside
metal
Tightly stuck, needs more energy to escape
Inside
metal

48. Each photon has: Energy = Planks constant * Frequency
Typical energies
Photon Energies:
Each photon has: Energy = Planks constant * Frequency
(Energy in Joules) (Energy in eV)
E=hf=(6.626*10-34 J-s)*(f s-1) E=hf= (4.14*10-15 eV-s)*(f s-1)
E=hc/l = (1.99*10-25 J-m)/(l m) E= hc/l = (1240 eV-nm)/(l nm)
Red Photon: 650 nm
Ephoton = 1240 eV-nm = 1.91 eV
650 nm
Work functions of metals (in eV):
Aluminum
4.08 eV
Cesium
2.1
Lead
4.14
Potassium
2.3
Beryllium
5.0 eV
Cobalt
5.0
Magnesium
3.68
Platinum
6.35
Cadmium
4.07 eV
Copper
4.7
Mercury
4.5
Selenium
5.11
Calcium
2.9
Gold
5.1
Nickel
5.01
Silver
4.73
Carbon
4.81
Iron
Niobium
4.3
Sodium
2.28
Uranium
3.6
Zinc
4.3