Third Polarizing Filter Experiment Debunked (2004)

The Third Polarizing Filter Experiment Demystified - How It Works

© Copyright 2004 Darel Rex Finley. All rights reserved. This article, with illustrations and copyright notice intact, may be freely distributed for educational purposes.

Pass light through two polarizing filters that are oriented at 90° to each other, and no light passes through. But place a third filter between them, 45° to each of the existing filters, and surprisingly, some light gets through!

This popular experiment is often described as "strange". It is usually presented in the context of quantum mechanics, as an example of the "spooky side" of quantum effects. Rarely, however, does the presenter inform the audience that this experience can be explained in very simple terms of cause and effect, without reference to creepy quantum magic or anything like that.

Let's start by reviewing the standard test:

Figure 1

In Figure 1, a parallel unpolarized light source is emitted through a polarizing filter, and the light registers strongly in a light meter at the other end.

Figure 2

In Figure 2, a second filter is introduced, oriented at 90° to the first. Now no more light comes through.

Figure 3

In figure 3, a third filter is placed between the first two, at 45° from each of them. Suddenly, the light meter registers a significant amount of light, but not as much as in Figure 1. Scary!

Fear and the word "filter"

Why do these results seem scary? The reason is due to the misapplication of the word "filter". A filter is generally understood as a device that removes certain elements from a stream, while leaving others virtually untouched. A good example of a filter is a sieve: it blocks objects of a particular size, while allowing objects of other sizes to pass.

Figure 4

Another example would be a color filter that eliminates certain frequencies of light while letting others pass.

Figure 5

Understood in this way, the results of the polarizer experiment are indeed frightening. If the all-blocking equivalent of Figure 2 is constructed using sieves or color frequency filters (see Figures 4 and 5), we are certainly confident that adding additional filters in the middle of the sequence will not give different results in the end.

But what if our so-called "filters" could not only block stream components, but also modify them? Then, we wouldn't be at all surprised if adding new "filters" in the middle made the elements go all the way to the end. If a sieve could not only block particles but also change their size, or if a color filter could not only block frequencies but shift light to a different frequency, then all bets are off.

That's actually what a polarizer does.

Figure 6

Look at Figure 6 and ask yourself this question: What percentage of unpolarized light is oriented at exactly 0° or very close to 0°? Almost nothing - certainly less than 1%. So if a polarizer simply removed the unwanted orientations, the strength of the remaining light would be almost entirely gone - it would be less than 1% of the strength of the original light source. But you know a polarizer doesn't do that, because you can pick up a regular pair of polarizing sunglasses and observe that they aren't even particularly dark! Obviously, something else has to happen.

Figure 7

Let's see what happens to each of the orientations shown in our simple diagram (from Figure 6):

Figure 8

In figure 8, we see that the light already at 0° is unchanged. We already knew that.

Figure 9

In Figure 9 we see what happens to the eastern light...