Light’s Dual Nature

Today, Charles K. Kao was named a winner of the 2009 Nobel Prize in Physics for his pioneering work on fiber optics used in communications. In the 1960s, Kao determined how to transmit light over 100 km in fiber optic cable when the standard at the time was only about 20 meters. With that opening, I thought I would relate how my understanding of the physics of light has matured over the last few years.

I studied Electrical Engineering both as an undergraduate and as a graduate student. As an undergraduate, I took a class in solid state physics, and we studied things that, well, had to do with solid state physics. Anyway, one thing I do remember learning in the class is that light can act both as a particle and as a wave. This is one concept I never understood at all until recently. It took 25 years for me to run across the explanation I needed to begin to understand this concept.

For some strange reason, I recently read every popular book I could find on astrophysics, relativity, quantum mechanics, and related concepts (yes, I am a geek). One thing I often encountered in my reading was the double slit experiment, which demonstrates the dual wave and particle natures of light and other quantum (very small) particles.

In this well-known experiment, quantum particles (electrons are often used, but photons achieve the same result) are emitted at two slits in an otherwise impervious plate, and the particles that pass through the slits are recorded on a screen beyond the plate. If one slit is covered, a particular pattern of particle impacts is recorded on the screen. If the other slit is covered, again a particular pattern of impacts is recorded on the screen but offset from the first results. All is as expected. See below for a double slit plate typical of those used in this experiment.

Image from WikiMedia Commons.

However, and this is where it gets interesting, when both slits are open, and particles pass through both slits, the pattern on the screen beyond the slits is an interference pattern (wait a minute), and not just the summation of the results of each single slit experiment. An interference pattern is generated when two waves interact. So, now the quantum particles are acting in some way like waves (tricky little buggers) and interfering with each other.

But wait, there is more. If particles are emitted one at a time at two open slits (someone developed an emitter that emits electrons one at a time), the interference pattern is still generated. What this very strange result means is that a particle must in some way be interfering with itself. Each electron passes through one and only one slit, but apparently that electron’s probability wave simultaneously passes through both slits, ultimately affecting where that electron lands on the screen.

Image from WikiMedia Commons.

See above (click to enlarge) for time lapse images of a screen used in a double slit experiment utilizing a one-at-a-time electron emitter. Image “e” clearly shows an interference pattern. Note that each electron still arrives at a single point on the screen.

Hitachi provides a description of their version of the double slit experiment as well as a video showing their results.

Wikipedia has a rather extensive article on this subject.

And finally, for a really clear explanation of the double slit experiment, watch the following video. It is pretty good until the last minute or so. Ignore the “act of measuring” effect, as this has been largely disproved.

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