User:Boleszek/Notebook/Physics 307l, Junior Lab, Boleszek/2008/12/12: Difference between revisions
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[[Image:h_e_apparatus.jpg|right|thumb|Figure 1. A schematic of the apparatus used to measure the stopping voltage of emitted photo-electrons. See text for details]] | [[Image:h_e_apparatus.jpg|right|thumb|Figure 1. A schematic of the apparatus used to measure the stopping voltage of emitted photo-electrons. See text for details]] | ||
Light from a mercury lamp (Pasco OS-9282 Hg light source) is refracted through a triangular prism and bands of light are shined onto a photo-diode tube (Pasco AP-9368 h/e Measuring Apparatus) containing a material with a low work function. The photo-diode tube and its associated electronics have a small capacitance which becomes charged by the photoelectric current. When the potential on this capacitance reaches the stopping potential of the photo-electrons, the current decreases to zero, and the anode-to-cathode voltage stabilizes<small><sup>4</sup></small>. This final voltage between the anode and cathode is therefore the stopping potential of the photo-electrons. A standard voltmeter (Wavetek true RMS DMM) is used to measure the voltage. | Light from a mercury lamp (Pasco OS-9282 Hg light source) is refracted through a triangular prism and bands of light are shined onto a photo-diode tube (Pasco AP-9368 h/e Measuring Apparatus) containing a material with a low work function. The photo-diode tube and its associated electronics have a small capacitance which becomes charged by the photoelectric current. When the potential on this capacitance reaches the stopping potential of the photo-electrons, the current decreases to zero, and the anode-to-cathode voltage stabilizes<small><sup>4</sup></small>. This final voltage between the anode and cathode is therefore the stopping potential of the photo-electrons. A standard voltmeter (Wavetek true RMS DMM) is used to measure the voltage. | ||
===A. Measurement of Charge Time=== | ===A. Measurement of Charge Time=== | ||
[[Image:chargingtime.jpg|right|thumb|Figure 2. graph of charge time vs. intensity with error bars indicating the standard deviation of the mean at each point.]] | [[Image:chargingtime.jpg|right|thumb|Figure 2. graph of charge time vs. intensity with error bars indicating the standard deviation of the mean at each point.]] | ||
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Five data points are collected at each intensity for both yellow and violet. The data is averaged for each intensity, errors are calculated, and the results as seen in Figure 2 are plotted using MATLAB. These procedures are documented in the MATLAB [[Physics307L:People/Osinski/Photoelectric/Matlab Code|code]] section entitled "Charge Time". | Five data points are collected at each intensity for both yellow and violet. The data is averaged for each intensity, errors are calculated, and the results as seen in Figure 2 are plotted using MATLAB. These procedures are documented in the MATLAB [[Physics307L:People/Osinski/Photoelectric/Matlab Code|code]] section entitled "Charge Time". | ||
For those who are interested in replicating our experiment, we provide our raw data | For those who are interested in replicating our experiment, we provide our raw data in the Appendix. Detailed notes explaining the difficulties we ran into along with our process of solving them can also be found there. | ||
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We shine light on the photo-diode tube and wait until the DVM reading has stabilized. The first set of measurements is shown below | We shine light on the photo-diode tube and wait until the DVM reading has stabilized. The first set of measurements is shown below | ||
<pre> | <pre> | ||
UV Violet Blue Green | UV Violet Blue Green Yellow | ||
Max(V) 2.044 1.71 1.49 .847 with | Max(V) 2.044 1.71 1.49 .847 with .716 w/filter | ||
</pre> | </pre> | ||
[[Image:spectra_order.jpg|right|thumb|Figure 4. A diagram of the 1st, 2nd, and 3rd order spectra of the mercury light refracted through the triangular prism. Overlap of the 2nd and 3rd orders is shown. A table displays the frequencies of each visible component of the spectrum]] | [[Image:spectra_order.jpg|right|thumb|Figure 4. A diagram of the 1st, 2nd, and 3rd order spectra of the mercury light refracted through the triangular prism. Overlap of the 2nd and 3rd orders is shown. A table displays the frequencies of each visible component of the spectrum]] | ||
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100% - 1.505V 0.856V | 100% - 1.505V 0.856V | ||
</pre> | </pre> | ||
A noticeable trend of increasing V with increasing fq is observed. | A noticeable trend of increasing V with increasing fq is observed. We discuss why this occurs below in the Results and Discussion section. | ||
==II. Determination of h== | ==II. Determination of h== | ||
[[Image:4graphs_Vvsfq.jpg|right|thumb|Figure 5. 4 graphs of stopping voltage vs. frequency with least squares fits which are used to determine h and ω by analytic and interpolation methods.]] | [[Image:4graphs_Vvsfq.jpg|right|thumb|Figure 5. 4 graphs of stopping voltage vs. frequency with least squares fits which are used to determine h and ω by analytic and interpolation methods.]] | ||
We have four sets of data to analyze | We have four sets of data to analyze (Figure 5.); two sets for the first order spectrum and two sets for the second. It should be noted that after considering the possible reasons for the existence of an unexpectedly high stopping voltage for the green band of the second order spectrum (discussed later) we redid those measurements and obtained values of .842V for the first run and .849V for the second run. These values will be used in the analysis | ||
Using MATLAB we are asked to make plots, perform linear least square fits, and determine h and ω_o for each of the four data sets. We scale the Voltage data by e (electron charge) so we can deduce from the equation | |||
:<math>eV=h\nu-\omega_0</math> | :<math>eV=h\nu-\omega_0</math> | ||
that the slope is h and the y-intercept is ω_o. | that the slope is h and the y-intercept is ω_o. | ||
The standard deviation of the mean of our result is propagated according methods explained in Chapter 8 of ''An Introduction to Error Analysis by John R. Taylor. I explain how Taylor's method is applied to the data in the Appendix. | |||
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#[[Lab Notes]] | #[[Lab Notes]] | ||
#[[Procedure for Error Propagation]] | #[[Procedure for Error Propagation]] | ||
#MATLAB [[Physics307L:People/Osinski/Photoelectric/Matlab Code|code]] | |||
==Conclusion== | ==Conclusion== | ||
Revision as of 17:04, 13 December 2008
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Measurements of the Photoelectric Effect Used to Calculate Plank's ConstantBoleszek Osinski, Darrell Bonn
AbstractAs one of the simplest yet most conclusive observations of quantum phenomena, the photoelectric effect is an excellent subject for undergraduate experimentation. We recreate this famous effect in order to measure the value of Plank's constant. We follow the procedure presented in Prof. Gould's manual1 to measure the charge time of the capacitor in an h/e measuring apparatus and the maximum stopping potential for various intensities and frequencies of light emitted by a mercury lamp incident on a photoelectric material. The light is separated into individual spectra with a prism, but effects of overlapping spectra are noted. A discussion on the imperfections of the measuring device is presented along with a thorough error analysis of the measurements. We find a linear relationship between frequency of light and voltage produced by the measuring apparatus which fits the formula [math]\displaystyle{ eV=h\nu-\omega_0 }[/math]. With a final result of 7.1(48)*10^-34Js we conclude that our experiment confirms the photoelectric effect.
I. Introduction to The TheoryIt is arguably not an over exaggeration to say that Planck's constant is the most fundamental constant in quantum physics, because it arose from the earliest considerations of the quantization of energy. In response to the failure of the Rayleigh-Jeans formula to predict the finite energy density of a blackbody radiating at high frequencies (the ultraviolet catastrophe) physicist Max Planck decided to heuristically treat the energy as a function of frequency instead of representing it in terms of temperature. He could then treat the observed "hump" on the E density vs. fq graph as a version of the Boltzmann probability distribution. But Planck's great contribution came only when he realized that he could obtain the required cutoff (averageE → 0 as fq → ∞) if he modified the calculation leading from the probability distribution ((P(E)) to the average energy by treating the energy as if it were a discrete variable rather than a continuous one2. With this insight he converted the integral of E*P(E) to a sum and found that the energy had to be directly proportional to the frequency. This proportionality factor would later bear his name with the minuscule value h=6.626068*10^-34 J-s. Though Planck had been one of the first physicists to employ a discrete mathematics in describing radiation he was unsure whether or not energy was actually quantized or not. In a letter to R.W. Wood, Planck called his postulate "an act of desperation" (Quantum Physics, Eisberg & Resnick, pg. 21). It was not until later observations were made that the quantization of energy was thought to be a natural phenomenon and not just a clever mathematical interpretation. Among the most important of these observations was the fact that when light shined on a certain material electrons were emitted whose kinetic energies were independent of the light intensity and proportional to the light frequency. It was Albert Einstein, who was an early proponent of the reality of Planck's postulate even before Planck himself believed in it, who bravely made the step to arrive at a surprisingly simple linear formula (KE= hv + w) relating stopping potential to light frequency1. The determination of the slope and y-intercept of this formula will be the aim of the following experimental procedure. It should be noted that the accepted value quoted above was not obtained from a photoelectric experiment. The value is actually obtained from the Watt balance experiment, the gyromagnetic ratio of a shielded proton, the von Klitzing constant (from the quantum Hall effect), and the Josephson constant (from the Josephson effect), and the weighted mean of these results is reported as the accepted value3.
I. Experiment Light from a mercury lamp (Pasco OS-9282 Hg light source) is refracted through a triangular prism and bands of light are shined onto a photo-diode tube (Pasco AP-9368 h/e Measuring Apparatus) containing a material with a low work function. The photo-diode tube and its associated electronics have a small capacitance which becomes charged by the photoelectric current. When the potential on this capacitance reaches the stopping potential of the photo-electrons, the current decreases to zero, and the anode-to-cathode voltage stabilizes4. This final voltage between the anode and cathode is therefore the stopping potential of the photo-electrons. A standard voltmeter (Wavetek true RMS DMM) is used to measure the voltage. A. Measurement of Charge Time We measure the time it takes for the voltage reading of the h/e apparatus to reach a chosen value of cutoff voltage for two different frequencies of light. Each of the two spectral lines are sent through a variable transmission filter with intensities of 20%, 40%, 60%, 80%, and 100%. Measurements are performed with Yellow (5.18672E14Hz) and Violet (7.40858E14Hz) light incident upon the slit that opens up to the photoelectric material. Five data points are collected at each intensity for both yellow and violet. The data is averaged for each intensity, errors are calculated, and the results as seen in Figure 2 are plotted using MATLAB. These procedures are documented in the MATLAB code section entitled "Charge Time". For those who are interested in replicating our experiment, we provide our raw data in the Appendix. Detailed notes explaining the difficulties we ran into along with our process of solving them can also be found there.
B. Measurement of Stopping Voltage vs. Frequency We shine light on the photo-diode tube and wait until the DVM reading has stabilized. The first set of measurements is shown below UV Violet Blue Green Yellow Max(V) 2.044 1.71 1.49 .847 with .716 w/filter  We proceed to make two measurements of stopping potential of the first and second order light bands, resulting in four total, which are shown in Figure 4. The raw data for these measurements is provided in the Data Page and primary thoughts on the data are in the Lab Notes. The observed linear behavior satisfies the energy relation for the photoelectric effect
but only after an anomalous effect due to the overlap of unseen spectra in the UV range is noticed and eliminated. Our solution to the problem of overlapping spectra is provided in the appropriate section below.
C. Measurements of Maximum Voltage vs Intensity of LightWith light shining upon the slit we measure the maximum voltage reached at consecutively higher intensities for Violet and Green spectra. Violet Green 20% - 1.487V 0.846V 40% - 1.485V 0.850V 60% - 1.489V 0.852V 80% - 1.486V 0.852V 100% - 1.505V 0.856V A noticeable trend of increasing V with increasing fq is observed. We discuss why this occurs below in the Results and Discussion section. II. Determination of h We have four sets of data to analyze (Figure 5.); two sets for the first order spectrum and two sets for the second. It should be noted that after considering the possible reasons for the existence of an unexpectedly high stopping voltage for the green band of the second order spectrum (discussed later) we redid those measurements and obtained values of .842V for the first run and .849V for the second run. These values will be used in the analysis Using MATLAB we are asked to make plots, perform linear least square fits, and determine h and ω_o for each of the four data sets. We scale the Voltage data by e (electron charge) so we can deduce from the equation
that the slope is h and the y-intercept is ω_o. The standard deviation of the mean of our result is propagated according methods explained in Chapter 8 of An Introduction to Error Analysis by John R. Taylor. I explain how Taylor's method is applied to the data in the Appendix.
IV. Results and DiscussionI have presented graphs next to their respective sections above so I will not repeat them here.
After calculations and plots are completed I notice that the intensity vs. charge time plot for violet exhibits an unexpectedly high charge time at 20% intensity along with the largest error bar, so it is reasonable to ignore that point. The overall standard deviation of the mean including this stray point is 0.2423s Without it the standard deviation of the mean reduces to 0.2036s This is the value that I choose to present as the final result. Our results show that as the intensity of light increases the time required to charge the capacitor in the h/e apparatus decreases, though this relationship does not appear to be linear as we previously thought it would. This actually makes sense because a capacitor does not charge linearly over time, but quickly at first and slowly at the end, following a negative exponential determined by the physical properties of the material and the magnitude of current coming its way. Our graph actually appears to be approximating a negative exponential of some kind. Though I cannot be sure at the moment what is the exact formula for this graph I can comment that it follows very nicely from the quantum theory of light. Were we to imagine millions of photons colliding with correspondingly millions of electrons in a material we would expect more electrons to be emitted when more photons are incident upon them (as long as the photon energy is large enough). More electrons correspond to higher current, which in turn correspond to lower charge time because the capacitor becomes saturated more quickly. A skeptic might ask why it is that the charge times for each intensity are shorter for higher energy (yellow) light than lower energy (violet) light when we would expect that emission current should only be proportional to intensity, not energy. I would respond that a higher energy light causes electrons to be emitted at higher velocities, so even though a cross-section of the yellow and violet light induced electron beams would have the same density of electrons at the same intensity, the electrons from the yellow light travel from the photoelectric material to the capacitor in a shorter time than do those emitted by the violet light. I could also comment on the skeptic's incredulity in expecting an undergraduate student to rigorously defend quantum physics.
The observed behavior indicates to us that the frequency dependence of the stopping voltage is linear. Thus our experiment supports a quantum theory of light because a wave's energy is proportional to its intensity (and I recall that it is actually Intensity^2) but our measured voltages are not. If we assume that the quantum theory is correct then electrons are "knocked out" of atoms in the material because individual particle-like photons collide with them with an energy larger than their bonds. Then we would expect that even a single photon of sufficient frequency will impart enough energy on a bound electron to emit it with the same energy as a million photons would a million electrons. This expectation is confirmed by the observation that a lower intensity of light results in a lower emission current, which simply corresponds to lower number of emitted electrons, because the lower intensity light contains fewer particle-like photons.
We notice that the green band results in a much higher stopping potential in the 2nd order than in the 1st. After speaking to Prof. Koch about this anomalous behavior we come to the conclusion that there must be an unseen factor at play. When we scan the h/e apparatus across the green band we notice that the voltage remains high even when we are in an apparent “dark” region on either side of green. When we cover the slit with both green and yellow filters we find that the stopping voltage of the green line is 0.842V! This result is very close to the measurement of the first order green line so we choose to use it instead of the out of place 1.242V. A second measurement of the second order green stopping voltage yields 0.849V. We have reason to believe that whatever unseen light made the voltage rise to 1.79V has been filtered out by this combination of filters (we also notice that the yellow filter does most of the filtering). After rereading the procedure it would be appropriate to add that the overlap of higher order UV spectra with lower order green is alluded to in section 5.3.1 of Prof. Gould's manual Darrell has the idea that his reading glasses might work as a good UV filter rather than the yellow filter (since his glasses have clear lens UV filters). When he placed a lens over the slit we did in fact witness a stopping voltage of 0.847V for the second order green band.
We find a trend towards increasing voltage as intensity increases, but the quantum theory of light predicts that stopping voltage should only increase as a function of increasing frequency, not intensity. Rather than assuming that we have found evidence against the photoelectric effect, we believe the rise in voltage with intensity occurs because the photodiode exhibits an imperfection consisting of a current leakage. At lower intensities the total current in the apparatus is lower than at higher intensities so the leakage constitutes a larger fraction of the lower intensity current and is therefore more noticeable to the DVM. Thus the voltage reading from the unfiltered light most accurately reflects the actual kinetic energy of the emitted electrons because the current leak is small compared to the emission current.
Here are the results for the four data sets: h1 = 7.1921e-034Js, stdm = 2.9322*10^-36Js || ω1 = 1.6097eV, stdm = 0.0123eV h2 = 7.1812e-034Js, stdm = 7.2352*10^-36Js || ω2 = 1.5954eV, stdm = 0.0304eV h3 = 7.2338e-034Js, stdm = 2.9684*10^-36Js || ω3 = 1.6246eV, stdm = 0.0125eV h4 = 7.1433e-034Js, stdm = 6.0527*10^-36Js || ω4 = 1.5807eV, srdm = 0.0254eV The average h and omega are: h = 7.1876*10^-34Js, stdm = 4.7971*10^-36Jsω_o = 1.6026eV, stdm = 0.0202eVIn the Teacher's Guide section the Pasco manual4 performs the experiment on obtains h = 6.6406E-34 (0.22% off), Wo = 1.412 eV. AppendixConclusionSJK 04:58, 8 December 2008 (EST) Note, Paul found a PASCO help page which stated the expected value of the work function (it was quite low but I don't remember the exact number...check for Paul's notebook. The source of the systematic error will be a great thing to explore tomorrow. In your final version of the report, part of this section will become your "conclusions" and part of it will be in the "results and discussion" section.
AcknowledgmentsI thank Prof. Steve Koch for teaching me the proper way to analyze data and propagate error as well as for stimulating us to think about the peculiarities of the h/e apparatus current leakage. Thanks also goes out to Mr. Darrell Bonn, whose experience in scientific instrumentation allowed us to quickly pinpoint reasons for inconsistent measurements and who was instrumental in the setup of the experiment. =ReferencesSJK 04:27, 8 December 2008 (EST)You will need some references to original peer-reviewed research in your final version. You can move me and Darrel to the acknowledgments section (which you will need to create)...anything in the references section should be specifically mentioned at least one time in the text of the paper or a figure caption.
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