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Published: 13 January 2021

• optics
• oscillations & waves

This week we’re exploring the physics of polarized light! We have several demonstrations of polarization in our collection; two of the most popular are perhaps the most straightforward: M7-03, which consists of two polarizing filters (or polarizers) and a light source; and M7-07, which adds a third polarizer.

Light is an electromagnetic wave, made up of oscillating electric and magnetic fields. We call a wave polarized when this oscillation has a particular orientation as the wave travels through space. The direction of the electric field defines the direction of polarization of the wave.

You can see two polarizers in action in this video starring Prof. Manuel Franco Sevilla.

A polarizer like this, also called a polarizing filter, passes only light of a given linear polarization. So it acts as a filter; if the first one is polarized vertically, it will block any horizontally polarized component of the light, and pass only the vertically polarized components. When the two polarizers are in line (which is to say that their axes of polarization are aligned), the second polarizer has very little effect on the light passing through. The first polarizer creates linearly polarized light; the second one, with the same polarization, passes nearly all the light that came through the first one. If we rotate the second polarizer, though, the axis of polarization, the direction in which it requires light to be polarized in order to pass through, rotates. So when the second polarizer is out of line with the first polarizer, it is only passing whatever component of the light from the first polarizer is also in line with the second one. As they rotate farther apart, that component is reduced. Once the two polarizers are fully 90 degrees apart, they no longer have any component in common, so together they pass no light at all! If the first one is polarized entirely vertically, and the second is polarized entirely horizontally, they are perpendicular.

Which is an important aspect of physics that this demo shows: that linearly polarized light can be treated as having separable components, just like we can separate the component vectors of linear motion of an object in space, and a polarizing filter passes only light components parallel to its polarization. So if we add a third polarizer, canted with respect to the other two, it can pass components parallel to its axis of polarization, however we choose to orient that. This can have some interesting results, as we see in the next video, starring Dan Horstman.

 The passage of light depends on the orientation of the current wave’s polarization and the filter it encounters – so adding the third filter actually could allow more light to pass!