Wednesday, April 28, 2010

A Study of the Mysterious Nature of Light

In the 17th century, physicists such as Newton and Hooke debated whether light were a particle or a wave. Decisive experiments around the turn of the 18th century resolved these debates conclusively in favor of the classical wave theory. But in the early part of the twentieth century, experiments that studied the photoelectric effect required light to also have particulate properties. Theoretical explanations to resolve these disparate pieces of evidence gave birth to the modern quantum theory of light, where particles may act, in some sense, as both waves and particles simultaneously.
This project presents a number of experiments that demonstrate, first, why the wave theory was accepted in classical physics; second, why light must sometimes be considered a particle; and third, how the interference associated with the classical wave theory occurs even when there is only a single "particulate" photon in the apparatus at any one time. The only known explanation for these behaviors is quantum mechanics.

To demonstrate that light exhibits the wavelike property of interference, you may perform updated versions of classical experiments (Young's Slit and Poisson's Spot Experiment). Then using a single photon detector, you can show that light exhibits particulate properties. Finally repeat some classical experiments at very low light levels using the photon detector to measure the interference patterns.

The classical experiments show the interference patterns indicative of the wave like properties of light. However, the single photon detector was able to show that photons are detected as individual particles. Repeating the classical experiments, we observed that, even at photon counting rates so small that only one photon is in the apparatus at any one time, the "wave" interference patterns are still observed. In a sense, the photons seem to be interfering with themselves.

We observed that light exhibits both wavelike and particulate properties. However, neither of these properties can be those of our common sense perceptions of them. The only known explanation of this is the theory of quantum mechanics, which predicts the behavior of light very well, but doesn't allow us to feel comfortable saying we "understand" light.


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