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sec1 - I Experimental Evidence for Quantum Mechanics...

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I. Experimental Evidence for Quantum Mechanics Quantum mechanics (QM) is a tool that was invented to explain experimental results. It is nothing more and nothing less than that. The utility of QM is therefore based entirely upon its ability to predict and explain experimental results, and by this measure it is a phenomenal success. There has yet to be an experiment of any type that violates the basic principles of QM. Thus, to begin with, we should discuss some of the experimental results that illustrate key principles of QM. Since this is a chemistry course, we will slant our perspective towards chemically relevant experiments, but similar effects can be found in any situation where the systems are small enough and the temperature is low enough. a. Polarization of Light Light waves can be polarized in any direction perpendicular to the direction of motion of the wave. So, for example, if we have a laser propagating in the z direction, the light beam can be polarized either along x or y . In this sense, light can be thought of as a transverse wave (i.e. one whose oscillations are perpendicular to the direction of propagation) and the two polarization directions can be thought of as: These two polarization components can be separated using a polarization filter . Typically, the filter consists of a crystal composed of rows of aligned molecules. Then, light whose polarization is not aligned with these rows will not pass through the crystal; meanwhile, light whose polarization is aligned with the crystal axis will be able to pass through the gaps between the rows. x y z x y z and
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Light from typical sources (such as a lamp or the sun) is not polarized; if you pass it though a polarization filter, some of the light passes though, and some does not. We will depict this simple experiment by: Where the round circle represents a polarization filter, and the vertical lines indicate that it is a polarization filter in the x direction. The polarization filter performs a simple measurement ; it tells us how much of the light is polarized in a given direction. This measurement is, however, very boring. It gets interesting when we start to consider multiple polarization measurements being applied to one laser beam. For example, if the first filter is x while the second filter is y , we get no light transmitted: x y Expt. 1 To put it another way, the first filter measures the polarization of the light and tells us that a certain part of the wave is x -polarized. Then, the second filter measures how much of the resulting x -polarized beam is actually y -polarized. The obvious result of this experiment is that none of the x -polarized light is simultaneously y -polarized. This makes sense from a physical perspective (none of the x - polarized waves would fit between the y -oriented slits) and also from
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a logical point of view (it is impossible to have the light polarized in two mutually exclusive directions).
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