LED Photoelectric Effect Experiment Apparatus designed by Wayne Garver, University of Missouri, St. Louis

Experiment description by Rex Rice

Albert Einstein’s interpretation of the photoelectric effect was central to the development of the photon model of light and 20th century physics. You will perform a photoelectric effect experiment using the apparatus pictured to the right and use Einstein’s interpretation to find Planck’s constant. You will need the photoelectric apparatus, a set of light emitting diodes (LEDs), a light shield, made from a cardboard box to cover the phototube and LED Bracket, and two digital multimeters

200 microamp

The diagram above shows the colors and wavelengths of the light emitting diodes used in this experiment. The photo to the left shows what’s inside the black box and the electrical connections. Note that the 9-volt battery on the left is the source used to create the stopping potential for the experiment. The D-cell holder contains four D-cells which provide a potential difference of 6 volts to the light emitting diodes.

Setting up and testing the equipment Start by checking the apparatus to make sure that the batteries are viable. Connect the red LED to the LED holder. The long lead on the LED should be inserted into the socket connected to the white wire and the short lead in the socket connected to the black wire. If properly inserted, the LED should light assuming the battery is good. If the LED doesn’t emit light the batteries could be disconnected from the holder or might be dead. You’ll have to open the case of the apparatus to check the batteries (D-cells). Then connect the multimeter that will be used for measuring the stopping potential, set to be used as a voltmeter in the 2-volt DC range with the red port associated with the stopping potential on the PE apparatus connected to the high potential port on the multimeter. The negative port on the PE apparatus (black or white) should be connected to the common port on the multimeter. Turn the switch to the on position and rotate the stopping potential knob clockwise. The values should range from 0 to about 1.7 volts as you rotate the knob clockwise from its most counterclockwise position to its most clockwise position. If you are not getting potential difference readings, the 9-volt battery inside the box may have to be replaced. Note that the equipment should be stored with the switch in the “down” position and the stopping potential knob all the way counterclockwise. Finally, connect a second multimeter to the photocurrent ports. Make sure that the meter is set to the 200 mV DC range and that the red port on the PE apparatus is connected to the V/Ω port on the multimeter and the black (or white) port on the multimeter is connected to the common port on the multimeter. Note that there is a 100 kΩ resistor between the two ports for measuring the photocurrent. Since there will only be a potential difference across the resistor if there is a current in it, measuring the potential difference across the known resistor would allow you to calculate the current through it. Since we are attempting to provide a stopping potential that is sufficient to stop the photoelectrons, we will adjust the stopping potential until the potential difference on the photocurrent meter reads zero.

UV 405 nm Blue 463 nm Aqua 505 nm Green 525 nm Yellow 593 nm Orange 605 nm

Red 631 nm IR 880 nm IR 945 nm

Performing the experiment With both multimeters connected and set to the appropriate settings, plug the shortest wavelength LED into the socket with the long lead to white, short lead to black. If this is the UV LED, it should glow violet. Turn the stopping potential switch to the off position and the stopping potential knob all the way counterclockwise. Cover the vacuum tube with the light blocking box so that the hole in the box lines up with the hole in the LED support. Then carefully insert the LED through the hole in the box until it is in the hole in the LED support. You should now read a potential difference with the photocurrent voltmeter. With the knob turned completely counterclockwise, turn on the stopping potential switch (up position). Slowly turn the stopping potential knob clockwise, noting the changes in the photocurrent potential difference. When the photocurrent voltmeter reads zero, you have reached the stopping potential for the highest kinetic energy photoelectrons. Record the stopping potential associated with the point at which the photocurrent just went to zero. Also record the wavelength of the LED. Repeat this procedure for each of the LEDs in your kit. The IR LEDs emit no visible light, so you will either need to use the spectrometer or the Radio Shack infrared LED tester to verify that you have the LED inserted correctly and the LED is emitting radiation. When you have measured and recorded the stopping potential for each of the nine LEDS, along with their wavelengths, you are done collecting data. Analysis of Data Calculate the frequency of each LED. Plot a graph of stopping potential in volts vs. frequency in hertz. Note any data points corresponding to LEDs that did not cause a photocurrent. Plot those points, but don’t include them in the curve fit. Including them will alter the slope and make it more difficult to determine the photoelectric threshold (cuttoff) frequency. What is the significance of the slope of this graph? What is the significance of the x-axis intercept? Calculate the maximum kinetic energy of the ejected photoelectrons, and plot a graph of maximum kinetic energy in joules vs. frequency in hertz. What is the significance of the slope of this graph? What is the significance of the x-axis intercept? If you understand the physical significance of the slope of each of your graphs, you should have no trouble determining what value to compare the slope to for the purpose of doing error calculations. Conclusions In your summary/conclusions for this experiment, describe the photelectric effect. Then explain the predictions that correspond to a classical electromagnetic wave interpretation of the incident light and the discrepancy between the predictions and the observed behavior. Then explain Einstein’s hypothesis concerning the nature of light, and his interpretation of the results of the photoelectric effect experiment based on the observed results. Include an energy analysis that results in the photoelectric effect equation. Why might Einstein’s use of Planck’s quantum hypothesis in his interpretation of the results of the photoelectric effect be worthy of a Nobel Prize?