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Physics 30 Unit C UA pt
C: Quantum Physics
Name: __________
Date: __________
1. In the early 20th century, physicists debated about whether light was a particle or a wave.
Create a table which summarizes the evidence that light is a wave, the evidence that light is a
particle and the evidence that light has both wave and particle qualities.
2. In an alternate universe, Mr. Langdale asks Ms. Arnason to buy him a green LASER and she
says YES!!! The wavelength of the light is 532 nm and it has a power rating of 4.00 mW. How
many photons are emitted from this LASER in 12.0 seconds? (1.28 x 1017)
3. If electrons in a cathode-ray tube are accelerated by a potential difference of 8.50 x 103 V,
what is the maximum frequency of the emitted X-rays? (2.05 x 1018 Hz)
4. While experimenting with the photoelectric effect, students project a form of EMR at the
cathode of a photocell, but no photoelectric current is observed in the circuit. Describe two
changes to the experiment the students could make in order to observe a current and explain why
these changes would produce current.
5. A group of students investigate the particle nature of light by shining light of set wavelengths
onto a metal surface and measuring the resulting flow of electrons. Their data is shown below:
wavelength of incident
light(nm)
680
590
520
440
current (pA)
0.0
0.0
8.0
8.0
Explain why current was observed after 520 nm and why the current did not increase when the
light was changed to 440 nm.
6. The students then apply a potential difference to the apparatus, forcing the electrons to remain
at the cathode. Their data is again shown below:
wavelength of incident
light(nm)
520
440
400
380
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stopping
voltage (V)
0.11
0.53
0.75
0.92
4/26/2013
a) Using the data given, complete this new data table for frequency of incident light and
maximum kinetic energy of the photoelectrons.
maximum kinetic energy (J)
frequency of the incident light (Hz)
b) On the grid below, make a graph of KEmax vs. f. (Or, make the graph on your calculator and
sketch the line of best fit on the graph below).
c) From the graph, state the work function of the
metal, the threshold frequency and the
experimental value of plank’s constant. (W = 3.6 x
10-19 J, f = 5.5 x 1014 Hz, h = 6.0 x 10-34 Js)
7. Light with a frequency of 5.85 x 1015 Hz produces a photoelectric current in a photocell
circuit. If it takes 12.8 V to reduce the photocurrent to zero, what is the threshold frequency of
the metal cathode? (2.76 x 1015 Hz)
8. LD wishes to run faster (as if that were even possible), so he gets students to shine different
colours of light on him as he runs, theorizing that the photons hitting him will give him extra
momentum. In order to reach the fastest speed, which colour of light should LD’s students
bombard him with? Explain using appropriate equations.
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4/26/2013
9. In 1923, Arthur Compton proved Einstein’s theory that photons had momentum in the famous
Compton Experiment. A diagram of the experiment is shown below:
θ = 40o
Before Collision
λinitial = 14.2 x 10-12 m
pelectron intial = 0 Ns
After Collision
λfinal = ???
pelectron final = ???
From the information above, determine:
a) the momentum of the initial photon. (4.67 x 10-23 Ns)
b) the new wavelength of the scattered photon. (1.48 x 10-11 m)
c) the momentum of the scattered photon. (4.48 x 10-23 Ns)
d) the momentum of the scattered electron. (3.19 x 10-23 Ns)
10. What is the wavelength of a 15 kg cannonball moving at 105 km/h? (1.5 x 10-36 m)
Bonus Questions: Answer only ONE!!!
Fair Bonus Question: The answer to question #10 is smaller than the Planck length. What is the Planck length and what does it represent?
Unfair Bonus Question: Discuss some ethical dilemmas inherent to bombarding LD with high frequency EMR.
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