Waves and Particles in Light
Luis A. Orozco
Department of Physics, University of Maryland
About the Lecture
The nature of light is a topic that has animated discussions in physics since the time of Isaac Newton. The argument of wave versus particle is resolved in quantum electrodynamics by a formalism that combines both of these aspects. The formalism is fundamentally statistical, and as with quantum phenomena in general, it is through the statistical uncertainty—fluctuations—that the wave and particle natures of light sit self-consistently side by side. Recent experiments show this with renewed clarity paving the way to new questions about the possibilities of manipulating light.
About the Speaker
LUIS A. OROZCO is a Professor in the Department of Physics at the University of Maryland. His interests are in Quantum Optics, Laser Spectroscopy, and tests of discrete symmetries in atoms. Previously, he had taught physics at the State University of New York at Stony Brook. He had also been a Research Visitor at the National Institute of Standards and Technology. He holds a bachelor's degree in electrical and mechanical engineering from ITESO in Guadalajara, Mexico and a Ph.D. in physics from the University of Texas. He did postdoctoral work in physics at Harvard. He is a member of the American Physical Society and the Optical Society of America.
President Robert Hershey called the 2,187th meeting to order in the Powell Auditorium of the Cosmos Club at 8:22 pm February 11, 2005.
The minutes of the 2,186th meeting were read and approved.
Mr. Hershey introduced the speaker of the evening, Mr. Luis A. Orozco. Mr. Orozco is a professor in physics at the University of Maryland and a distinguished traveling lecturer of the American Physical Society.
Why, Mr. Orozco began, should we be concerned with the question of whether light is a wave or a particle? This question has bothered many people. A first approach is to compare it to sound, which is known to be a wave. A solid object, sound flows around it, blocks a light beam. He demonstrated these phenomena. Therefore, light is a particle.
But then he showed the data from a photon counting experiment. Each photon was represented by a click, or a dot on paper. When there are only a few dots, they look like noise. As the frequency is increased, they start to exhibit oscillations. Is light sometimes a wave and sometimes a particle, he asked?
He turned to Euclid of Alexandria for an earlier perspective. Euclid said the eye sends out probes to feel an object. Close objects look large and distant ones small because the nearer objects intercept more of the probes, Euclid reasoned. This idea stood as truth for many centuries.
Only about 1000 years ago, this idea was tested by Abu Ali al Hasan Ibn al Haytham. He asked his colleagues to look at the sun. They soon said it was impossible. Al Hasan said that showed that optics were emissions, not tactile probes. This is believed the first known example of the scientific method, considered by many the greatest idea of all time.
Six hundred years later, Newton advocated a corpuscle theory. Rays of light, Newton said, were corpuscles emitted by objects. They were light's “least parts.” He called it “manifest” that light consists of parts.
But particles have collisions, and light beams do not deflect each other. Newton was also concerned about an Island crystal that deflected light in two different directions. He posited that the particles had spin, causing some to go one way and some the other.
Huygens disagreed. If light were made of particles, two beams crossing would cancel each other. He argued that light was a wave. Waves need a medium. Huygens's answer was the hypothetical concept, ether.
In 1810, Malus discovered that light is polarized by reflection. He noticed, rotating the Island spar crystal, that the light was extinguished in certain positions. He argued that light particles have sides or poles. Mr. Orozco demonstrated the blocking of light using two polarized filters. Each passed light; the two together blocked all light.
Thomas Young beamed light through two small slits and observed a number of dots. This is an interference pattern. Using a diffraction grating produces the same thing, but the spots are far more spread out. You can check it out with a laser disk. Interference patterns represent waves.
In the days of Napoleon, a prize was offered for the most interesting and amazing piece of physics. Fresnel submitted a paper on the wave properties of light. Poisson and Arago were on the jury. Poisson deduced from Fresnel's paper that there should be a dark spot in the diffraction pattern with a bright spot in the center of it. He thought this was so unlikely he was ready to dismiss Fresnel. Arago, however, tried it. He found the bright spot, and Fresnel won the competition. This settled for a long time the question. Light is waves.
James Clerk Maxwell, in the1860's, combined electricity, magnetism and light into one theory electromagnetic theory. He said light was an electromagnetic wave.
Then Heinrich Hertz demonstrated that electrons could be emitted from metal if a certain light hit it. This is the so-called photoelectric effect.
The Michelson experiment demonstrated there was no ether. Michelson had studied in Napoli and moved to the Naval Observatory here in Washington. Here his boss showed him a letter from Maxwell inquiring if it was possible to measure carefully the speed of light at different times of the year. That question fired his imagination and led to his famous experiment.
Max Planck reluctantly brought particles back to light. Invited to a colleague's house for coffee and cake, he was shown an experiment in measurement of spectra. That night he wrote a post card to Heinrich Rubens. Rubens wrote back. The result of the post card was the birth of the quantum.
Einstein said if light is a particle, that could explain the photoelectric effect of Hertz.
Bohr began to understand the hydrogen atom with the quantum. Later he established the complementarity principle.
Louis de Broglie found that material particles are also waves. Incidentally, his is the only Nobel Prize based on a paper that fit on half a page.
The birth of the photon probably happened in a paper by Stark in 1907. He found that the quantum of light also has mass.
Paul Dirac reconciled quantum mechanics and special relativity with quantum electrodynamics. This is perhaps the most successful, most thoroughly tested, theory we have in this area.
Richard Feynman explained the quantum in terms of trajectories of particles that keep track of their phase, like a clock. Quantum mechanics was back in shape.
Recently, John Bell posed question that have to be answered by alternatives to quantum mechanics. Bell died 15 years ago.
Mr. Orozco described some of his own studies. Using light trapped between two mirrors, he used a detector to start a clock and another to stop it to time the intervals between clicks. Averaging over 913 of these yields a wave picture.
Light is a wave and (!) a particle, he concluded. The solution is the uncertainty intrinsic in the measurement. Only quantum electrodynamics solves the problem.
He offered to answer questions.
"How does light go through a vacuum?" one person asked. It is self-propagating, Orozco said. The electric and the magnetic part stimulate each other producing a cycle.
How does light know up-down from left-right? It does not. Light has just two orthogonal directions, it does not know which is which. What determines which way it goes is the source.
Mr. Hershey announced the next meeting. He made the parking announcement. Finally, he adjourned the 2,187th meeting to the social hour.
Ronald O. Hietala,