The frequency of light tells us how many times that light wave oscillates up and down in 1 second. Polarization tells us the direction the light is oscillating. There are three major groupings of light polarization: linear, circular, and elliptical. Linearly polarized light is the most common light we use in spectroscopy, so we aren’t going to talk about circularly or elliptically polarized light here. However, there are types of spectroscopy experiments that do use circularly polarized light. If you are curious about them, image search the phrases to find some neat animated gifs showing light with different polarizations moving through space. For now, we will focus on linearly polarized light.
Fig. 1 Examples of oscillating electric fields propagating along the z-direction. The left picture shows horizontal linear light while the right picture shows vertical linear light .
Linear polarization is light that is oscillating up and down in a single plane. There are two specific types of linearly polarized light: horizontal and vertical. Horizontal light oscillates . . . horizontally with respect to some surface, and vertical light oscillates . . . vertically to that surface, see Fig. 1 Remarkably, we can use these two polarizations to describe any linearly polarized light. For example, suppose the light was oscillating at a 45◦ angle. We would describe the light as half horizontal and half vertical.
In the lab, there are optical devices called half-waveplates, sometimes written as λ/2 plate or just λ/2. A half-waveplate can rotate the linear polarization of light. If you have horizontally polarized light, you can use a half-waveplate to change the polarization so that it is 10% vertical and 90% horizontal, 50% vertical and 50% horizontal, 75% vertical and 25% horizontal, etc. You can even make the light exiting the half-waveplate be completely vertical. That might seem like a neat trick, but the real usefulness comes when we put a second optical device after the halfwave plate called a polarizing beam splitter (PBS), see Fig.2.
Fig. 2 Using a half-waveplate and a polarizing beam splitter, we can create two beams of light. You can rotate the half-waveplate to control the ratio of light in each path. In normal spectroscopy setups, you are looking down on the light and optics from above. In this orientation, p-polarized light is horizontal, indicated by the blue arrows and s-polarized light is vertical, indicated by the dotted circles .
By tradition, the light that bounces off the PBS is called s-polarized light while the light that passes through is called p-polarized light.1 This is because the PBS can technically be in any spatial orientation. For safety reasons, we almost always keep the light in a horizontal plane. In this typical setup, s-polarized light is vertically polarized light while p-polarized light is horizontally polarized light. If you were to rotate the PBS so that the s-polarized light was going straight up (don’t do this, it is an eye hazard!!), that s-polarized light is now horizontally polarized while the p-polarized light is now vertically polarized. This is why we use “s” and “p”.
In the lab, we use half-waveplates and polarizing beam splitters to split a single laser beam into two beams and also control the power in each of them. If the waveplate is oriented such that the light polarization leaving the waveplate is completely vertical, all of the light would bounce off the PBS. If the polarization of the light leaving the waveplate was at 45◦, there would be equal power in both lasers leaving the PBS.
1 Fun fact: the “s” stands for the German word “senkrecht”, which translates to perpendicular; the “p” stands for the German word “parallel”, which translates to parallel (that isn’t a misprint, the German word for parallel is parallel). I like to use “s” for skip and “p” for pass-through.
