Thomas Young performed his famous “double slit experiment in 1801. The results of this experiment demonstrated the dual nature of light. The term “dual” in the dual nature of light means that light, which is electromagnetic radiation, has both wavelike and particle properties. While the initial experiment was done with light it has since been repeated with electrons, atoms, and even molecules as large as 60 carbon atoms called a Buckyball. All of these experiments show the same dual nature results.
Common terminology
Before we get into the actual experiment and its results lets discuss some of the vocabulary. The first term we should discuss is quantized. If something is quantized then it exists only in discrete continuous amounts. Light is quantized into quanta called photons. Quanta is the fundamental size unit the smallest, indivisible portion of a property. A small packet or chunk of light, for example, which can’t be broken down any further.
Next we need to discuss some terminology about waves. These terms can be used with water waves, light waves, sound waves, basically any type of wave. There are different types of waves, transverse and longitudinal, that both have the same features. The amplitude of a wave is the height of a wave and is directly proportional to the energy of the wave. The higher the wave, the greater the amplitude. Greater amplitude corresponds to a more energetic wave. The top of a wave is called the crest and the low point of a wave is called the trough.
Diffraction is the term used to describe how waves bend around an opening or barrier.
Courtesy of s-cool.co.uk
Wave interference
Waves can interact with other waves or even with itself and this is called interference. There are two basic types of interference. When the peak also called crest of one wave interacts with the crest of another wave then constructive interference occurs. If the trough of one wave interacts with the trough of another wave then the same constructive interference occurs. Constructive interference results in a larger single wave, as if the two parts of the wave that interacted constructed a new single larger wave. If the crest of one wave interacts with the trough of another wave then the waves cancel one another out in a process called destructive interference.
Wave Function
The wave function in quantum mechanics is an equation that describes all the characteristics of a particle.
The square of a wave function represents the probability of finding a particle at a given time and place. The square of the wave function becomes significant when trying to determine the location of an object during the double slit experiment.
The Experiment
Thomas Young designed and conducted an experiment which had shocking results. He was attempting to show that light behaved as waves rather than the accepted notion that light behaved as individual particles. Young knew that waves created interference patterns when they interacted with each other. To test his hypothesis he shined a light source at a barrier with two slits which were fairly close to one another. The light passed through the slits and shone on a screen behind the barrier. If light was acting as a wave he would expect to see an interference pattern on the screen. If light was acting as a particle he would expect to see an image of the the two slits as the particles passed through the barrier.
The Results of the experiment
When Young performed his experiment an interference pattern emerged on the screen behind the barrier. This seemed to confirm Young’s idea that light behaved as a wave. In the early part of the 20th century quantum mechanics was born. Scientists like Max Planck, the father of quantum mechanics and Albert Einstein argued that light behaved as both a particle, the photon and a wave. The double slit experiment was done again only this time scientists were able to fire photons one by one at the barrier. In this case one would think that the pattern that emerged would not be an interference pattern but rather a band of light at through each slit. Think of spraying paint though a two slit barrier. Bands would only be present at the site of the slits, similar to a stencil.
This experiment has been repeated many times since Young’s experiment. Scientists have even repeated the experiment using electrons which they could fire one at a time to try to determine wave versus particle behavior. Even when the electrons were fired one by one, eventually an interference pattern emerged. When the electrons are fired one at a time they can not be interacting with other electrons to produce an interference pattern. This means the electrons are interacting with themselves to create an interference pattern.
Here is where it gets weird. In order to for an interference pattern to occur the barrier must have two slits. How can a single electron be aware of another slit? One solution to this question is that the electron splits and goes through each of the two slits and interacts with itself. Another alternative is that the electron does not split but goes through both slits at the same time. Both of these scenarios vary greatly from our everyday experiences dealing with Newtonian mechanics.
With advances in technology we can now perform the experiment using either wave or particle detectors to once and for all determine if an electron behaves as a particle or a wave. Quantum weirdness, however, rears it head once again. If we try to use a detector to see which route, that is which slit, the electron went through, then no interference pattern is detected and the result is consistent with particle behavior. The act of observing, in this case by use of a detector, destroys the interference pattern by collapsing the wave function. The implication is that the act of observing which slits the electrons goes through causes the interference pattern to collapse and the particle pattern to be demonstrated. If, however, no detector is used then the wave interference pattern prevails. These results demonstrate the dual nature of many things once thought of as particles and that observation at the quantum level can have a dramatic impact on the very results one is tying to measure.
Neville’s Phantastic Physics Phun 