PHYS 1407 – Conceptual Physics II
Photoelectric Effect I
Threshold and Intensity
Leader: _________________________ Recorder: __________________________
Skeptic: _________________________ Encourager: ________________________
Materials
Planck’s Constant Apparatus
Mercury Lamp
2 x DMM’s with red and black banana cables instead of leads
Small Lab Jack
Red Diode laser with power supply
Green Diode laser
Index card
Flashlight
Safety
1. The mercury lamps used in this experiment emit significant amounts of UV radiation. Do not look directly into them.
Introduction
The photoelectric effect is probably the most important experiment in establishing the quantum nature of light. In the photoelectric effect light shines on a metal surface and electrons are emitted. The electrons emitted in the photoelectric effect are often referred to as photoelectrons. In this activity, we will explore some of the features of the photoelectric effect.
Procedure
Part
I – Preliminary Observations
1. Examine the apparatus
There is a door on the top of the Planck apparatus that you can open to view inside. Open the door and peek inside.
Q1. Describe the appearance of the tube inside. Illustrate your description with a sketch.
Q2. There are two prominent pieces of metal inside the tube known as electrodes. The light enters the window through the side. Which of the electrodes does the light mainly shine on? Explain why you think this is the case. This electrode is known as the photocathode.
Q3. Which of the electrodes then will collect the photoelectrons?
Gently close the door on top of the Planck Apparatus.
2. Set-up the DMM’s
On one of the DMM’s if not already there, plug the red banana cable into the 200 ma socket and the black banana cable into the COM socket on the DMM. This DMM will be referred to as the ammeter for the remainder of this activity. Plug the other end of the red banana cable into the socket labeled Ammeter + and the black banana cable into the socket labeled Ammeter - on the Planck Apparatus.
On the other DMM plug the red cable into the socket labeled V-Ω and the black cable into the socket labeled COM. This DMM will be referred to as the voltmeter for the remainder of this activity. Plug the other end of the red cable into the socket labeled Voltmeter + and the black cable into the socket labeled Voltmeter – on the Planck’s Apparatus.
Have the instructor verify that you have set up the DMM’s correctly.
3. Set-up the Light Source
Place
the mercury discharge lamp on the lab jack.
Adjust the height of the jack so that the lamp is at the height of the
window on the side of the Planck Apparatus.
Don’t plug the lamp in yet.
4. Zero the Current
Turn on the ammeter to the 200 μA
DC setting. Turn on the voltmeter to the
2
Part II – Observation with Green and Red lasers
Part II will be conducted with the lights off. Once everyone has completed part II, the lights can be on for the remainder of the lab.
Note for the following when the current is 0, the meter may drift around small values near zero like -1 μA to 1 μA, or so. This is normal and you should take that into account when determining whether the ammeter reads 0 or not.
5. Shine Green Laser Light on the Photocathode
Once the lights are out, open the door on top of the Planck Apparatus. Because the ambient level of light has changed in the room, the current may not read 0. Readjust the current offset knob so that the current reads 0 again.
Shine the green diode laser so that it illuminates the photocathode.
Q4. Does the ammeter now measure a current? If so record the amount.
Q5. Did the current show up instantaneously or did it take a while for it to appear?
Turn off the diode laser.
Q6. When the diode laser is turned off, does the current persist or go back to 0?
Q7. If the current went back to 0, did this take a while or did it occur instantaneously?
6. Shine Red Laser Light on the Photocathode
The current may have drifted. Readjust the Ammeter Zero Adjust knob so that the current reads 0 again, if necessary.
Shine the red diode laser so that it illuminates the photocathode.
Q8. Does the ammeter now measure a current? If so record the amount.
Q9. If no current appears initially does a current appear after the laser has illuminated the photocathode for a while?
Once everyone is to this point, the room lights can come back up.
Q10. What is the difference between red and green light?
Q11. Does light always produce photoelectrons when shined on the photocathode?
Q12. In this case were photoelectrons produced when higher or lower frequency light shined on the photocathode?
Q13. Explain the observations in terms of a threshold frequency of light above which photoelectrons are emitted and below which they aren’t.
Q14. If the red laser has a wavelength of about 650 nm and the green laser has a wavelength of about 525 nm, where must the cutoff wavelength lie for whether or not photoelectrons are emitted?
Part
III – Observations with the Mercury Lamp
4. Data Acquisition
Plug in the mercury lamp. It will take a few seconds for it to warm up. The window through which the light enters is covered by filters that only allow certain colors of light to pass. The filters are chosen to coincide with strong emission lines from mercury. The four wavelengths chosen are 390 nm, 430 nm, 540 nm, and 580 nm.
P15. Will photoelectrons be emitted when 390 nm light shines on the photocathode? Explain.
P16. Will photoelectrons be emitted when 430 nm light shines on the photocathode? Explain.
P17. Will photoelectrons be emitted when 540 nm light shines on the photocathode?
Explain.
P18. Will photoelectrons be emitted when 580 nm light shines on the photocathode?
Explain.
Try each filter in turn. The filter labeled UV-39 will transmit 390 nm light, the filter labeled V-Y43 will transmit 430 nm light, the filter labeled V-054 will transmit 540 nm light and the filter labeled V-058 will transmit 580 nm light. You change filters by rotating which filter is in front of the window.
D19. Record your results for each filter in the table below.
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Filter |
Current |
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Part IV – Intensity
Rotate to the filter that gave the strongest photocurrent in D19. Slowly insert the index card between the lamp and the filter and observe the photocurrent.
Q20. Does the photocurrent change as you insert the card? If so, in what way?
Q21. What is the ratio of the photocurrent when the window is unblocked and when the window is half-blocked.
Q22. What is the ratio of the intensity of the window when is unblocked and is half blocked?
Q23. How do the ratios that you found in Q21 and Q22 compare?
Q24. What does this suggest about the relationship between the intensity of the light and the current produced? Explain your answer.