Photoelectric Effect: Determining Planck's Constant

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Photoelectric Effect Lab


We used a photoelectric effect to determine the Planck’s constant. Using a mercury lamp, varying filters and varying voltages we measured the output current. We determined that the filters marked with higher numbers had lower voltages. Therefore, as the filter size decreased the overall voltages increased. Background/Introduction

Photoelectric effect is used to determine Planck’s constant in many experiments. Planck’s constant shows the size of quanta in quantum mechanics. We used a photoelectric effect apparatus to set up the experiment. We used four different filters and also recorded data for no filter at all. We recorded the output current in relation to the stopping voltage. The voltage increased and the current decreased in response.

Einstein used Planck’s theory of black-body radiation and later devised Planck’s constant. He stated that frequency multiplied by a constant would equal the energy in each quantum level of light. This experiment was completed to view the different effects from varying filters to see the relation of voltage and current.


First, set up the Photoelectric effect apparatus so that the aperture in front of the photodiode faces the mercury lamp. Next, connect the digital voltmeter to the red and black banana jacks on the top of the panel of the case. Turn on the mercury lamp and place the filter marked 436nm over the phototube aperture. Set the zero by turning the voltage adjust knob to a voltage just high enough to stop the current and adjust the zero adjust so that the meter reads zero. All photocells have some dark current so that zeroing with the aperture covered included the dark current in the zero position. If the current is brought to zero by adjusting the stopping voltage a true zero is obtained. If the zero drifts between readings, the radiation intensity on the photo-surface is too high and a phenomena known as fatigue is occurring on the photo-surface. If this is occurring, reduce the intensity by moving the source away from the aperture. Then, turn the “voltage adjust” known to its counter-clockwise limit, at which time the voltmeter should read zero of very close to it. Next, move the apparatus until the radiation is striking the center of the photodiode. The radiation intensity should be adjusted so that the meter is approximately 10 on the scale. Measure and record the output current as a function of the Stopping Voltage. The critical point on the curve is the voltage at which the current just falls to zero. Then, change the filter to the one marked 546nm and repeat the measuring procedure. Next, change the filter to the one marked 577nm and repeat the measuring procedure. Finally, change to no filter, which allows 406nm mercury line to fall upon the phototube and once again repeat the measuring procedure.


436nm| |
Stopping Voltage (Volts)| Current (A)|
0.01| 10|
0.047| 9|
0.096| 8|
0.123| 7|
0.202| 6|
0.254| 5|
0.297| 4|
0.382| 3|
0.544| 2|
1.917| 1|
546nm| |
Stopping Voltage (Volts)| Current (A)|
0.103| 10|
0.124| 9|
0.148| 8|
0.167| 7|
0.201| 6|
0.24| 5|
0.252| 4|
0.298| 3|
0.344| 2|
0.415| 1|
0.56| 0|
577nm| |
Stopping Voltage (Volts)| Current (A)|
0.033| 10|
0.056| 9|
0.088| 8|
0.124| 7|
0.156| 6|
0.175| 5|
0.216| 4|
0.262| 3|
0.29| 2|
0.388| 1|
0.942| 0|
406nm| |
Stopping Voltage (Volts)| Current (A)|
0.788| 10|
0.805| 9|
0.83| 8|
0.845| 7|
0.88| 6|
0.894| 5|
0.924| 4|
0.958| 3|
0.986| 2|
1.034| 1|
1.069| 0|

Calculations and Graphs


Through the course of the lab we learned about Planck ’s constant though the observation of the photoelectric effect in varying voltages of mercury light. We determined that as the light filter’s frequency increased the voltages increased as well....
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