Use the electric (E) vector of light to describe the behavior of plane polarized light.
Prior Knowledge and Skills
3.4 Electric and Magnetic Components of Light
Electric Vector (E)
Polarized Light (note: only need to watch linear polarization, to 6:00, but feel free to watch the whole video!)
Polaroid sunglasses are familiar to most of us. They have a special ability to cut the glare of light reflected from water or glass (see Figure 3.5.1). Polaroids have this ability because of a wave characteristic of light called polarization. What is polarization? How is it produced? What are some of its uses? The answers to these questions are related to the wave character of light.
Light is one type of electromagnetic (EM) wave. As noted earlier, EM waves are transverse waves consisting of varying electric and magnetic fields that oscillate perpendicular to the direction of propagation (see Figure 3.5.2). There are specific directions for the oscillations of the electric and magnetic fields. Polarization is the attribute that a wave’s oscillations have a definite direction relative to the direction of propagation of the wave. (This is not the same type of polarization as that discussed for the separation of charges.) Waves having such a direction are said to be polarized. For an EM wave, we define the direction of polarization to be the direction parallel to the electric field. Thus we can think of the electric field arrows as showing the direction of polarization, as in Figure 3.5.2.
To examine this further, consider the transverse waves in the ropes shown in Figure 3.5.3. The oscillations in one rope are in a vertical plane and are said to be vertically polarized. Those in the other rope are in a horizontal plane and are horizontally polarized. If a vertical slit is placed on the first rope, the waves pass through. However, a vertical slit blocks the horizontally polarized waves. For EM waves, the direction of the electric field is analogous to the disturbances on the ropes.
The Sun and many other light sources produce waves that are randomly polarized (see Figure 3.5.4). Such light is said to be unpolarized because it is composed of many waves with all possible directions of polarization. Polaroid materials, invented by the founder of Polaroid Corporation, Edwin Land, act as a polarizing slit for light, allowing only polarization in one direction to pass through. Polarizing filters are composed of long molecules aligned in one direction. Thinking of the molecules as many slits, analogous to those for the oscillating ropes, we can understand why only light with a specific polarization can get through. The axis of a polarizing filter is the direction along which the filter passes the electric field of an EM wave (see Figure 3.5.5).
Figure 3.5.6 shows the effect of two polarizing filters on originally unpolarized light. The first filter polarizes the light along its axis. When the axes of the first and second filters are aligned (parallel), then all of the polarized light passed by the first filter is also passed by the second. If the second polarizing filter is rotated, only the component of the light parallel to the second filter’s axis is passed. When the axes are perpendicular, no light is passed by the second.
Only the component of the EM wave parallel to the axis of a filter is passed. Let us call the angle between the direction of polarization and the axis of a filter θ. If the electric field has an amplitude E, then the transmitted part of the wave has an amplitude E cos θ (see Figure 3.5.7). Since the intensity of a wave is proportional to its amplitude squared, the intensity I of the transmitted wave is related to the incident wave by I = I0 cos2 θ, where I0 is the intensity of the polarized wave before passing through the filter. (The above equation is known as Malus’s law.)
3.5.1. Is the polarization direction of light parallel to a) the electric vector; b) the magnetic vector; or c) the direction of light propagation?
3.5.2. Is all light polarized?
3.5.3. How can you check the polarization direction of one polarizer by using a second polarizer?
Concept Check 3.5.1
Is laser light polarized or unpolarized? What is an important implication for this if you were trying to analyze an anisotropic mineral sample with a laser?
Want more information about polarization of light? See the OpenStax / Lumen learning physics textbook chapters referenced below.
Kevin Claytor. Polarized Light. (Nov 9, 2013) https://youtu.be/gP751qpm4n4
OpenStax, College Physics. OpenStax CNX. Mar 4, 2019 http://email@example.com.
Also located at: https://courses.lumenlearning.com/physics/chapter/27-8-polarization/
MIT OpenCourseWare. Laser fundamentals I: Polarization of laser light | MIT Video Demonstrations in Lasers and Optics. (June 15, 2012) https://youtu.be/mjwQTL6G8Fs