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LAB 11 Photoelectric Effect & Blackbody Radiation OBJECTIVES 1. Experimentally verify that the emission of photoelectrons depends only on frequency of the incident light and not the intensity of the light. 2. Predict & measure Planck’s constant h/e and the work function Φ/e of a metal. 3. Examine the blackbody radiation emitted by objects at different temperatures. EQUIPMENT UV light source, aluminum plate, electric pom-pom, plastic rod, wool, Photoelectric effect apparatus, DMMs, LEDs, PhET Photoelectric Effect Simulation. THEORY In photoelectric emission, light strikes a metal (cathode), causing electrons to get emitted. As a guide, use the following PhET simulation: http://phet.colorado.edu/en/simulation/photoelectric The classical wave model predicts several things about the photoelectric effect: (i) the total energy is spread across a wavefront striking the entire metal surface and nothing could happen until sufficient energy were absorbed. Therefore, no photoelectrons would be immediately ejected and one expects a time lag as to when light is absorbed and photoelectrons are ejected. (ii) Increasing the intensity increases the kinetic energy of the most energetic electrons (i.e. Kmax) would be emitted from the metal. (iii) Increasing the intensity would also increase the number of electrons emitted (or photocurrent). On the other hand, the quantum model predicts that (i′) photoelectrons are immediately ejected if the frequency is high enough assuming fincident > fthresold. (ii′ & iii′) Increasing intensity does produce a higher photocurrent but Kmax is independent of intensity. Furthermore, Einstein applied Planck’s theory and explained the photoelectric effect using conservation of energy: h f e e E hf K max eVstop Vstop where Kmax is the maximum KE of the emitted photoelectrons and is the work function, which is the metal’s binding energy. Plotting Vstop vs. f for different frequencies, the yintercept is equal to Φ/e and the slope is equal h/e. PROCEDURE Part 1: Photocurrent vs. incline voltage at different light intensities Predict the behavior of the photocurrent (ejected electrons) & stopping voltage a. Use the wave model to predict the following behaviors: Does the photocurrent immediately register a value when the light source is turn on or is there a time lag? Is the photocurrent higher for higher intensity levels? Is the stopping voltage higher for higher intensities or independent of intensity? b. Repeat (1a) using the photon model. Measure Photocurrent vs. Vincline c. Setup a data table with columns for Vincline(V) with values of (0, 0.2, 0.4, 0.6, 0.8, 1.0, Vstop), photocurrents iphoto(A) with intensity levels 1, 2, 3, and 4. d. Insert the blue LED into the photoelectric apparatus and cover the apparatus with the cover. Set the intensity level to 1, turn on the incline voltage & set to 0V, and measure the photocurrent. Increase Vincline to 0.2V, measure the new photocurrent, and record this value. Repeat this process until Vincline reaches the Vstop value. 1 e. Repeat (1d) for intensity levels 2, 3, and 4. Plot iphoto vs. Vincline for all four intensities on a single plot. f. Using your observations and the plot, answer the questions of (i) time lag, (ii) Vstop (Kmax) ∝ Intensity, and (iii) iphoto ∝ intensity , and based on the predictions of the Wave model Quantum model Which model better fits the data better: the wave or quantum model? Explain your answer using short concise sentences. Part 2: Predict and measure Planck’s constant h/e and the Work Function Φ/e a. Setup a data table with columns for LED description, wavelength, frequency, and stopping voltage Vstop. b. Set the intensity level to 4 (maximum intensity) and measure Vstop for the LEDs UV, blue, green, yellow, red and IR. a. Plot Vstop vs. frequency and use the Trendline function to display the line equation. Determine (h/e)expt and (Φ/e)expt from the line equation and write your values on the whiteboard. b. Compute the class average and uncertainty ((h/e)class ± 2σh). Does the accepted value of Planck’s constant fall within 2σh confidence interval, where (h/e)accepted = 4.14 ×10-15 V∙s? c. What is the work function/e ≡ (Φ/e of the metal (cathode) plate? Performing an online search of work functions, are there any metals with this value? Explain your reasoning. Part 3: The Photoelectric Effect and the Electric Pom-pom Photons hitting a metal surface (aluminum) will cause electrons to be ejected if the work function is less than the photon energy: Ephoton = hf > Φ. Will ultraviolet (UV) light cause electrons to be ejected from an aluminum surface? a. The UV light source produces two different wavelengths, 365 nm and 254 nm. Calculate the photon energy in electronvolts (eV) for each of these two wavelengths using hc = 1240 eV∙nm. b. The work function for aluminum is ΦAl = 4.28 eV. Predict if electrons will be ejected by the UV light source. Explain you’re reasoning in terms of Ephoton, ΦAl and an energy bar diagrams. Record all information in a table. c. Place the aluminum surface on top of the electric pom-pom and use a charged rod to scrap electrons from the rod onto the metal surface. When the strings of the pompom are repelled from each other, then the metal surface has an excess of electrons. i. Shine UV 365 nm light onto the aluminum surface; are electrons ejected from it? ii. Repeat (i) for UV 254 nm light. d. Did your predictions from part (1b) agree with what you observed? Explain your observations in terms of the photoelectric effect using short concise sentences. Part 4: Blackbody Radiation All objects above absolute zero emit electromagnetic radiation called blackbody radiation. As an object gets hotter, it emits more light and it emits light of higher energy (thus higher frequency and shorter wavelength). Warm objects emit mostly infrared light, IR, which we can’t see, but this special camera can “see” infrared light. In this station, you will observe the visible and infrared radiation emitted by three hotplates at different temperatures. a. Use the camera and your eyes to answer the following questions: Which emits more IR light – your eyes, or your nose? Why? Look at a cup of hot water vs a cup of cold water. Which one emits more infrared light? Why? 2 b. Look at the wire coil in the camera. Tap your foot quickly on the switch. Explain the types of light do you see? Now hold down the switch and let the coil get hot. Does it emit visible light? Is it still emitting infrared light as well? If we heat up something even more, what will happen to the wavelengths of light it emits? c. Does IR light passes through these materials (Predict first!): (i) Clear acrylic, (ii) glass, (iii) black plastic, and (iv) balloon. 3