Wave Nature of Light

1. Wave Nature of Light Experimental Apparatus One Photon at a Time Physics Group: Alex Pearson, Travis Christolear, Mei Mei Chan, Colin Smith Faculty Advisors: Vladimir Gasparyan, Thomas Meyer Objective: The properties of light have been variously explained by viewing light as a particle (Newton, Einstein, Compton) or as a wave (Huygens, Fresnel, Young, Maxwell). The main goal of this experiment was to study the quantum mechanical resolution of these two different viewpoints by demonstrating the wave-particle duality of light. Results In 1801, Thomas Young projected light through two slits onto a screen. His experiments showed that light behaves as a wave, putting into question preconceived Newtonian notions that light was a particle. In this experiment, each slit acts as a source of light, and the resulting pattern was explained using the constructive and destructive interference of waves. Fig. 7 Fig. 1 Double Slit Interference(1) The interference pattern created by this experiment produces bright maxima (Fig. 6) where the path length difference between the two slits is equal to an integer number, m, of wavelengths, λ, of the light(2) dsin = m Young’s experiment was reproduced using a diode laser (=670nm) as a source of coherent light and a photodiode that measures the intensity of the light across the interference pattern. Our results confirm the wave behavior and give the distance, d, between the two slits of 0.419mm. Fig. 8 Eq. 1 Fig. 6 Single and Double Patterns Troubleshooting Fig. 5 Double Slit Fig. 3 The Apparatus Fig. 2 Wave-Particle Duality Single Photon Experiment According to Richard Feynman, wave-particle duality "contains the only mystery of Quantum Mechanics"(4). In this experiment we demonstrate the wave-particle duality for photons. When light passes through a double-slit the characteristic interference pattern is observed, which can be explained by classical wave theory. When the intensity of light is reduced to such a low level that only one photon arrives at the two slits classical physics predicts that the photon must choose to pass through one or the other slit. While conducting various trials and experiments, an asymmetry was observed in the graph for the double slit interference pattern (Fig. 7) using the light bulb and the PMT detector, but not seen with the laser and photodiode detector. To study this asymmetry the double slit was reversed, inverted, and displaced. The results remained unchanged. After a series of other changes and additional trials, it was concluded that the PMT efficiency was not uniform across its surface. Particle Nature of Light Conclusion The results of the low intensity light bulb (λ=546nm) were plotted as a function of the detector slit’s positioning along the photomultiplier tube. The PMT output was measured using an electronic counter and then averaged over 30 seconds for each detector position measured in 0.05mm increments. The experiment was then repeated with each of the two individual slits covered to show that the light from the double slit was not just the sum of the light from the two individual sources (red and green curves in Fig. 7). To emphasize this result we plot in Fig. 8 the counting rate at the location of the central maximum and the adjacent minima. Classically, we would expect the counting rate at the center to be the sum of the counting rates for the individual slits, or about twice that at the adjacent minima. However, we observe a counting rate which is about three times larger. Using the equation from Young’s experiment (Eq. 1) the separation between the two slits was calculated to be 0.42mm. The main difficulties in this experiment arose when trying to align the light source, slits and detector, especially using the low intensity light. The random nature of when and where a photon would land on the detector gave a wide range of counting rates that had to be averaged over time. By the end of the nineteenth century, there were results that could not be explained using the wave theory of light. Einstein’s Nobel Prize winning explanation of the photoelectric effect(3) led to the reemergence of the particle theory of light. Our experimental apparatus (Fig. 3) allows us to perform measurements using single photons. The source is a variable light bulb with a selective filter that allows only green light and can have its intensity lowered to the point that only a small number of photons are passing through the apparatus at a time. The detector is a highly sensitive Photo-Multiplier Tube (PMT) that amplifies the signal from a single photon to a voltage that can be measured with an oscilloscope. Each signal is also output as a single TTL pulse which is electronically counted (Fig. 4). The speed of light (3x108m/s) is great enough that if photons were sent one at a time through the apparatus (approximately 1m in length), more than 300 million counts could be registered per second. When the experiment was performed, we collected an average of 400,000 counts per second. The PMT is only ~ 2-3% efficient, so this gives an actual rate of approximately 1 million counts per second, or 1/300th of that minimum rate. This gives confidence to the claim of the ability to perform the experiment with one photon at a time. The two experiments, using a high intensity laser beam and single photons from a low intensity light bulb passing through a double slit set-up, produce nearly identical results: a double slit interference pattern measuring the distance between slits of 0.420mm and 0.419mm, respectively. In the case of single photons, this leads us to conclude that a single light particle, or photon, is also a wave, which passes through both slits at the same time. Fig. 4 The Photon Counting Assembly • References: • http://www.assignmenthelp.net/assignment_help/young-double-slit-experiment.phpz • Hecht, Physics: Algebra/Trig, Brooks/Cole (1998) • Anderson, Introduction to Modern Physics, Saunders College Publishing (1982) • Feynman, Leighton, and Sands, The Feynman Lectures, Vol. III, Addison Wesley (1965) The Oscilloscope The Detector The Digital Counter