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2.4
Photoelectric Effect
Watch Simple Photoelectric Effect Demo http://www.youtube.com/watch?v=muxRZ1irsrk
Instructions: I would do the demo in class, but it is nice to have a video if a student is away.
1) By looking at the tinsel you can tell that it is
A) positive
B) negative
C) neutral
D) charged
It says the charge is negative, but by looking all we know is that it is charged.
2) The room light did not affect the charge. The sanitizing light did, because this light
A) is more intense B) is brighter
C) has a greater wavelength D) has a greater frequency
The sanitizing light has more energy and that is because it has a greater frequency.
Go to PhET: http://phet.colorado.edu/en/simulation/photoelectric
3) As the intensity of light is slowly decreased from 100% to 0% the rate of electrons emitted will
A) slowly decrease
B) suddenly decrease
C) become random
Does this demonstrate wave or particle behaviour of light? Explain.
It decreases and this decrease can be explained by either model. However, at really low intensity it gets
random which can only be explained by the particle model. When there are only a few photons, one may
hit and eject an electron almost immediately or after a very long wait. If the low intensity was due to a
small amplitude, then you would always have to wait a long time. Compare shaking fruit from a tree vs.
throwing rocks.
4) As the colour of light is slowly changed from ultraviolet to blue to red, the number of electrons will
A) slowly decrease in number B) suddenly decrease in number
C) become random
Does this demonstrate wave or particle behaviour of light? Explain.
There is a sudden decrease in the number of electrons if you go below the cut-off frequency. No matter
how intense you make the light, no electrons will be ejected. With a wave model, a low frequency would
mean longer waits but eventually the energy would build up and eject electrons. Compare shaking fruit
from a tree with a low intensity vs. throwing tiny pebbles.
5) The electrons are emitted with a variety of energies. The kinetic energy of the fastest electrons can be
determined by seeing what voltage is needed to stop them.
E
a) Fill in the following table and graph the results.
V (V)
PhET
-19
E ( x 10 J) qV
200
300
400
500
600
nm)
f ( x 10 Hz) c/
f
Instructions: Students should do a sample calculation, predict the graph. Then let PhET do the work.
E=
f=
b) What is the slope? Planck’s constant.
c) How will this graph change if you change metals? Why?
The slope will not change because it is a universal constant. The line will still be straight but
shifted, changing the intercepts. Each metal holds onto the electrons with different strengths. The
vertical intercept is a measure of the binding energy – called in this case the work function.
d) What is the equation of the line and what is the physical meaning of each term? E = hf -W
The E refers to the kinetic energy of the fastest emitted electrons. The horizontal variable is the
frequency of the photon and the vertical intercept is determined by the metal. The equation is an
example of the conservation of energy.
e) What is meant by the work function and the threshold frequency?
They are the vertical and horizontal intercepts of the E=hf graph for the photoelectric effect. The works
function is the minimum energy needed to remove the electron from the attraction of the metal. The
threshold frequency is the lowest frequency that can do this and the two are related by h.
Note: Einstein provided this particle-like explanation for light in 1905 and for this work he received the
Nobel Prize. (His work on relativity was too controversial.)
6) How are the photoelectric effect and an LED similar/different?
The photoelectric effect involves a metal and an LED involves a semiconductor. Otherwise they use
reverse processes, where one electron kicks out one photon or the reverse. The photon only appears
before or after, but the electron doesn’t disappear, it just gains or loses energy.
7) How do these two devices compare to solar panels and light detectors?
A solar panel or light detector are also made of a semiconductor and are more precisely the opposite of
an LED.
8) The interference and diffraction of photons and electrons is similar and the deBroglie equation is true
for both. However, there are also some key differences.
Charge: Electrons have a negative charge of 1.602 x 10-19 C.
Speeds: Photons always travel at c in a vacuum regardless of frequency or energy.
Higher energy electrons travel at faster speeds but always less than c.
Mass: An electron has a set mass and a photon has none even though it has momentum.
Momentum: The momentum of both is h/ but the momentum for an electron is also mv
Energy: The energy of a photon is given by hf and for an electron it is ½ mv2 - until you get to
relativity.
9) How has quantum technology made TV watching easier?
The remote control signals with an ir LED that is detected by light detector – both are quantum and you
don’t need to get out of your seat!
10) How does the photoelectric effect demonstrate the key concepts of quantum physics?
Wave-Particle Duality: It has a frequency but the energy comes in packages – photons.
Measurement-Disturbance:??
Intrinsic Randomness: At very low intensities the arrival of the next electron is random.
Uncertainty: ???
Textbook:
12.1
p. 604 # 8 – 14
11) How is the photoelectric effect related to the Compton Effect?
If higher energy photons are directed toward a substance, there is so much energy that the electron
cannot absorb it all and so a lower photon is also emitted. This interaction of the two photons and the
electron conserve momentum and kinetic energy just like a billiard ball hitting a stationary one.
Historically, many physicists found this effect to be the most convincing argument for photons – more
so than the Photoelectric Effect and blackbody radiation..