Photoelectric Effect (1) (1)

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Physics

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Dec 6, 2023

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Name Anthony Cervantes Date 11/01/2023 Class PHYSICS_011 Photoelectric Effect Purpose To study the photoelectric effect and understand the connection between the wavelength and energy of the incident light and photoelectrons emitted. Connections to What You Already Know About in Life Did you know that we have many types of technology used today that function because of a phenomena that won Einstein his Nobel Prize? Solar cells, photodiodes and photoconductive devices all function because of a property that Einstein helped quantify and explain! Vocabulary Frequency, intensity, photoemission, wavelength Background Although Albert Einstein is most famous for E = mc 2 and his work describing relativity in mechanics, his Nobel Prize was for understanding a very simple experiment. It was long understood that if you directed light of a certain wavelength at a piece of metal, it would emit electrons, but light of some wavelengths wouldn’t emit electrons from the metal, no matter how intense or bright the light was. In classical theory, the energy of the light was thought to be based on its intensity and not its frequency or wavelength. However, the results of the photoelectric effect contradicted classical theory. Inconsistencies led Einstein to suggest that we need to think of light as being composed of particles (photons) and not just as waves. You will reproduce a photoelectric experiment and show that the energy (E) of a photon of light is related to its frequency and not its intensity. Procedure Section 1 1. Start Virtual Physics and select Photoelectric Effect from the list of assignments. The lab will open in the Quantum laboratory. 2. The laboratory will be set up with a laser shining at an angle on a sheet of sodium metal. Atoms in the metal absorb the energy from the light and emit electrons. The detector in the bottom corner detects the electrons that bounce off the metal. The intensity and wavelength of the laser can be adjusted. Record the initial intensity and wavelength of the laser in Question 1. 3. Turn on the detector by clicking on the red/green light switch on the detector. Answer Question 2. 4. Decrease the Intensity to 1 photon/second and note your observations about how the signal changes in Question 3. 5. Change the Intensity back to 1 nW and increase the Wavelength to 600 nm. Record your observations in Question 4. 6. Experiment with changing variables until you determine the maximum wavelength at which the emission of electrons occurs in the metal. Record your observations in Question 5. pg. 1 - Photoelectric Effect © Beyond Labz , all rights reserved

Name Anthony Cervantes Date 11/01/2023 Class PHYSICS_011 Questions 1. At what intensity and wavelength is the laser set? The intensity is at 1nW, and wavelength of the laser is set to 450 nm. 2. What does the signal on the phosphor screen indicate about the laser light shining on the sodium foil? The signal on the phosphor screen indicated that the electrons are being ejected from the sodium metal surface as well as the intensity and the wavelength of the laser. 3. How does the signal change when the laser intensity is decreased? What does this show about the relationship between the amount of emitted photons and the intensity of the incident light? When the lasers intensity is decreased, the center light starts flashing which means that when the intensity of the light decreases, the number of emitted photons will also decrease. 4. What do you observe on the phosphor screen when the wavelength of the laser is increased to 600 nm? No light is detected at the center of the phosphor screen as it previously did. 5. What is the maximum wavelength at which electrons are emitted from the metal? The maximum wavelength is 450nm for Na. Procedure Section 2 1. Click inside the Stockroom to enter the stockroom. Click on the clipboard and select the preset experiment Photoelectric Effect (Bolometer) . The intensity of the laser will be set at 1 nW and the wavelength at 400 nm. The detector used in this experiment is a bolometer and will be automatically turned on. This instrument measures the kinetic energy of electrons. Click the switch in the Bolometer display screen to switch from electron volts to Joules. You should see a green peak on the detection screen. The intensity or height of the signal corresponds to the number of electrons being emitted from the metal, and the x -axis is the kinetic energy of the electrons. Zoom in on the peak by clicking next to the peak and dragging the box that appears around the peak. 2. Hover over the top of the peak and record the kinetic energy of the electrons and the intensity in the data table in Question 6. The kinetic energy is actually displayed in 10 -19 Joule units, so record *10 -19 for every energy recorded. Increase the wavelength in 10 nm increments and record the kinetic energy and intensity of the peak for each new wavelength in the data table. When you hit the maximum emission wavelength, observe what happens as you continue to increase the wavelength. Answer Question 7. 3. Decrease the wavelength to a value where electrons are emitted. Observe what happens to the peak when you increase and decrease the intensity. You may need to zoom out to see the changes. Record your observations in Question 12. pg. 2 - Photoelectric Effect © Beyond Labz , all rights reserved

Name Anthony Cervantes Date 11/01/2023 Class PHYSICS_011 Questions 6. Record the wavelength (in nm) in the data table. Calculate the frequency (in Hz, or 1/s) and the energy (in J) using f = c λ and E = h*f where c = 3.0 ×10 8 m/s and h = 6.626×10 −34 J*s. Don’t forget to convert your wavelength units to meters. Remember 10 9 nm=1m. Wavelength (nm) Frequency (Hz) Laser Energy (J) Ejected Electron Kinetic Energy (J) Electron Intensity 400 7.5e+14 4.95e-19 0.3585 0.0499 410 7.30e+14 4.85e-19 0.2782 0.05 420 7.13e+14 4.73e-19 0.2008 0.05 430 6.95e+14 4.60e-19 0.1308 0.05 440 6.82e+14 4.52e-19 0.0682 0.05 450 6.64e+14 4.42e-19 0.0092 0.05 460 6.51e+14 4.32e-19 0 0.05 7. Are your observations of what occurs above the maximum emission wavelength consistent with what you observed before? Indeed, the ejected electron kinetic energy reaches maximum when the wavelength was set to 400nm, due to shorter wavelengths having higher frequency and higher energy. Therefore, shorter wavelengths also give ejected electrons more kinetic energy. 8. Make a graph of incident light wavelength vs. ejected electron kinetic energy from your data in the data table. Include a couple of the higher wavelength values that you observed. Graph the wavelength in nanometers on the x -axis and the kinetic energy in *10 -19 Joules (just plot the fractional part, not the power of ten) on the y -axis. pg. 3 - Photoelectric Effect © Beyond Labz , all rights reserved

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Name Anthony Cervantes Date 11/01/2023 Class PHYSICS_011 9. What does the shape of the graph show? What does it mean on the graph when the kinetic energy drops to zero? The graph shows the shorter the wavelength the higher the kinetic energy is. Which shows the line of the graph slanting towards the bottom because the wavelengths increase the shorter, they become. 10. Make a graph of laser energy vs. the kinetic energy of the ejected electrons from your data in the data table. Include a couple of the higher wavelength values that you observed in step 9. Graph the laser energy in *10 -19 Joules on the x -axis and the electron kinetic energy in *10 -19 Joules on the y -axis. 11. What relationship do you see between the energy of the incident light on the foil and the energy of the ejected electrons? pg. 4 - Photoelectric Effect © Beyond Labz , all rights reserved

Name Anthony Cervantes Date 11/01/2023 Class PHYSICS_011 The more energy of the light on the foil results in a much higher energy of ejected electrons. 12. What is the relationship between the peak and the laser intensity? The peak goes hand in hand with the laser and its intensity, the more energy to the incident light results into a higher peak of ejected electrons which is the relationship between the two. 13. Based on this experiment, explain why violet light causes photoemission of electrons but orange light does not. Which matters in the formation of photoelectrons: intensity or wavelength? Violet has always been known to have the highest frequency with the shortest wavelength rather than the orange light which produces less energy than the violet light. Different wavelengths are also present which is why they matter when it comes to the formation of photoelectrons. pg. 5 - Photoelectric Effect © Beyond Labz , all rights reserved

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