Physics 30 Unit C UA pt C: Quantum Physics
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) / current (pA)
680 / 0.0
590 / 0.0
520 / 8.0
440 / 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) / stopping voltage (V)
520 / 0.11
440 / 0.53
400 / 0.75
380 / 0.92
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 KEmaxvs. 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.
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:
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.