The University of Sydney School of Physics Ideas to Implementation The Photoelectric Effect Albert Einstein won his Nobel Prize in Physics for his insight in to the photoelectric effect. His understanding of this phenomenon was one of the milestones in the development of quantum theory and introduced the world to the concept of wave-particle duality. Photon electron A Current flows in circuit when light shines Waves? Particles? Waves and particles? Light definitely does behave like a wave: it diffracts, refracts and interferes, which are all wave properties. Light also seems to behave like a particle, like a lump: it can knock electrons off surfaces and transfer energy to other objects in packets of E = hf, just like a particle would. Observing the Photoelectric Effect In this experiment you can observe the photoelectric effect as certain frequencies of light knock electrons out of a metal surface. When the electroscope is charged, shine dim, ‘white’ light source (the desk lamp on ‘low’) onto the metal plate. Switch the lamp to its brightest setting. Do you observe any change in the charge on the electroscope? Predict Observe Explain The University of Sydney School of Physics Ideas to Implementation Now shine the UV lamp on the metal plate. What do you observe? Predict Observe Explain Measuring the photoelectric ‘stopping voltage’ In this experiment you again observe the photoelectric effect, but this time you’re going to measure the amount of energy the ejected electrons receive from the photons. energy E = hf. This means that electrons knocked out from the surface of a material will all have roughly the same amount of kinetic (moving) energy when they leave: Filter colour Yellow Green Blue Violet Ultraviolet Filter 5.19 x 1014 5.49 x 1014 6.88 x 1014 7.41 x 1014 8.20 x 1014 frequency (Hz) Stopping voltage (V) You now have the necessary information to determine the work function of the phototube’s cathode, and to find a value for Planck’s constant. How do you do this? • The electrons had energy K = hf – W when they left the surface. • Electrons passing through the voltage V applied to the phototube will lose an amount of energy equal to qv • At the stopping voltage, the electrons just don’t make it to the anode, they have lost all their initial energy — so for this voltage, qV = K. • This means qV = hf – W, which we rewrite as V = (h/q) f – (W/q) The University of Sydney School of Physics Ideas to Implementation This is a linear equation, similar to y = mx + c. So you can make a graph of V vs. f. It should be a straight line, with a slope equal to h/q and a y-intercept equal to W/q. Title ______________________________________________________ .