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Experiment #1 Photoelectric Effect February 13, 2014

The Photoelectric Effect

By: Yusuf Waxali

Lab Station #2Lab Partner: JohnFebruary 13, 2014

Photoelectric EffectObjectiveIn this lab, students will observe the line spectrum emitted by a mercury vapor lamp. Next, students will determine the most intense wavelength shining through colored filters. Finally, students will observe the photoelectric effect and record the stopping voltage for the three colored filters. From these results, students will calculate experimental values for , h, v0, and 0.

IntroductionCrucial to the study of modern physics is the concept of wave-particle duality, or the idea that something such as light may exhibit the behavior of a wave while being comprised of discrete particles. While light certainly does behave like a wave, undergoing reflection, refraction, and diffraction just as any other wave would, certain behaviors of light cannot be explained by wave properties alone.

For example, consider the photoelectric effect. When light shines upon a metal surface, some electrons are freed, producing a potential difference and thus electricity. This phenomenon cannot be explained by simple wave properties. Rather, particle behavior is required to make sense of the fact that light with certain wavelengths, regardless of intensity, can liberate electrons from certain metal surfaces. As Einstein postulated, the existence of photons, or the particles that make up light, was responsible for the instantaneous release of electrons from the target surface.

It is for this reason that the photoelectric effect is an essential part of understanding modern physics, which places much importance on the properties and behavior of light.

In The Photoelectric Effect Experiment, students will observe the colors on the mercury spectrum as ultraviolet light shines through a grating and on to a nearby piece of paper. After noting which colors are most visibly intense on the paper, students repeat with orange, green and blue filters. The respective wavelengths for each color light are provided. Students then collect data for voltage readings when light shines on the phototube in series connection with a voltmeter and an ammeter. After measuring the voltage of the circuit, which is tangible evidence for the photoelectric effect, students calculate values for the work function of the cesium antimonide phototube, Plancks constant, threshold frequency, and threshold wavelength.

By analyzing the numbers behind the photoelectric effect through calculation and experiment, The Photoelectric Effect Experiment captures the importance of the photoelectric effect and helps bring better understanding to the subject of Modern Physics.

Results

DataAfter observing the light from the Mercury vapor lamp as it passes through a grating, the colors were observed and described based on their relative apparent intensity. See Table 1.

Table 1: Intensity of Spectral Lines from Mercury Vapor Lamp. Shows the observed and actual relative intensities of each color of light, as well as the wavelength and eye sensitivity

Students then inserted orange, green, and blue filters and observed the colors that were transmitted through each filter. Each color of light was categorized as eliminated, transmitted poorly, or transmitted well. This process was repeated for each filter placed over the lamp. See Table 2.

Table 2: Effect of Filters. Shows the quality of transmission for each color of light through each of the three filters

After connecting the phototube, voltmeter, and ammeter in series, students measured the stopping voltage required to eliminate all of the current in the circuit. This process was conducted three times, one for each of the three filters. The wavelength () of the line responsible for the stopping voltage was found based off of conclusions drawn from Table 2, and the frequency (v) was calculated by dividing the speed of light by the wavelength. See Table 3.

Table 3: Recorded Stopping Voltages and Wave Properties. Shows the experimental stopping voltages for each filter as well as the wavelength and frequency for each of the filters

CalculationsPlease see the attached notebook paper with handwritten calculations.

ResultsAfter getting three voltage readings for each filter, a total of nine extinction voltage readings were plotted on a graph versus the frequency of each filter. The equation of the line of best fit is shown on the right of the graph, along with the slope of the line of best fit and the intercepts of the axes. See Figure 1.

Figure 1: Extinction Voltage vs. Frequency. Shows a scatter plot of all of the experimental values for the extinction voltage. Includes equation, slope, and intercept values for the line of best fit.

Finally, after collecting all of the data and plotting it, the intercepts and slope values allow the student to calculate h, , v0, and o using the following equations:

1. 2. For a table of the calculated values, see Table 4.

Table 4: Experimental Values. Shows calculated and published values for Plancks constant, the work function of CsSb, and the threshold frequency and wavelength for CsSb. ConclusionTo summarize, students experimentally found stopping voltages for a mercury lamp with colored filters on it and used this data to calculate experimental values for Plancks constant, the work function of CsSb, and the threshold wavelengths and frequencies of the phototube. The results for threshold frequency were acceptable, having the same order of magnitude as the published value. The experimental value for threshold frequency was 2.5E14 Hz. The work function was also acceptable, having the same order of magnitude as the published work function value. The experimental value for the work function was 2.78 eV. On the other hand, the experimental Plancks constant was flawed, varying from the published value by 7 orders of magnitude. The experimental value for Plancks constant was 1.78E-27 J*s.

One potential source of error was the use of the yellow filter rather than the green filter. This could have affected the results since the wavelength and frequencies were given for a green filter but the filter used was yellow. After omitting these data points and using only the voltage readings from the blue and orange filters, the new experimental value for h is 2.53E-34, which is much closer to the published value.

8Yusuf Waxali Physics 244