Light and Dark

What happens to light when it goes through polarized sunglasses?

If you have done the two previous experiments you have learned a bit about the wave nature of light. But light is a wave of what exactly? Picture an electron. It creates an electric field. The arrows are vectors that tell you the direction of the electric field and how strong it is. You can do the same thing with a magnet and a magnetic field. Light is a combination of the two types of fields. The electric field is pointing in one direction and the magnetic field is pointing 90 degrees to that. Normally when you see pictures of electric fields they are just sitting there, not moving. But with light, the electric field is moving like a wave and so is the magnetic field. Most of the time the electric field of different light rays are going in different directions. If you could see the direction of the electric fields of the light all the light rays all around you right now, they would all be going in different directions. When this is the case, the light is said to be "unpolarized." When light is polarized, it means that the electric field of every light ray is moving in the same direction. To make this happen, unpolarized light can be passed through a polarizer. A polarizer picks out the light rays with electric fields going in one particular direction. A polarizer acts like a filter and only allows light rays in one direction to pass through. The polarizers in your kit are actually clear plastic coated with a film of long chains of polymers. When light hits this, only light that has some part of its electric field vectors going in the direction of the polymer chains in the polarizer will pass through. This means that the light that comes out of a polarizer has electric fields all going in one direction. Imagine an arrow pointing at 45 degrees. If you were to draw this arrow on an x, y plot, you would need to know how much is going in the x direction and how much is going in the y direction. So the arrow has some x and some y components. Now think of this arrow as describing the direction of the electric field of a ray of light. Next, think of the polarizer as being in the x direction, so that only light with some x component can go through. Even though our 45 degree light ray isn't completely in the x direction, it has some amount in the x direction. Because it has a little bit in the x, that part can pass through the polarizer. The light that comes out the other side has all of its electric field in the x direction. Now what if this light were to try and go through a polarizer that was in the y direction it couldn't. Because the light has no component in the y direction it would be completely blocked. This is why light can't go through crossed polarizers. This experiment puts three polarizers in a line and looks at what happens when the middle one of the three is turned. The first and third polarizers are crossed but the middle one is free to move. You might assume that because the first and third polarizers are crossed that no light will make it to the end, but when you perform the experiment it becomes clear this is not the case. So what is happening here? Let's follow the light one step at a time. First light of all polarizations hits the first polarizer which picks out light in one direction, let's call that x. So when the light gets to the second polarizer it is all going in the x direction. This second polarizer is moved around during the experiment. When the polarizer is turned so that it is at least pointing a little bit in the x direction, some of the light can get through it. So now we move on to the third polarizer. At this point all the light is going in whatever direction the second polarizer is pointing. The third polarizer is pointing in the y direction. If the light coming out of the second polarizer is going even a little bit in the y direction then some of it will pass through the third polarizer and you can see it at the very end. So, basically, if the third polarizer's direction is pointing in a way that gives it both some x and y direction then light can get all the way through. The most light gets through when the second polarizer has the same amount of x and y. Many people think that laser light must be polarized, but that isn't true. As long as laser light is all "in phase" (see activity 1) then it is laser light. However, most laser light is polarized. Look at your laser through the polarizer and see if it is polarized.

• Real world connections
  • Based off of what you learned about polarization and by using these helpful resources, why do we wear 3D glasses when we go to the moves? How do 3D glasses work?
    • https://www.youtube.com/watch?v=-SMpGiNVymU
    • https://www.brainfacts.org/thinking-sensing-and-behaving/vision/2020/how-do-3d-glasses-work-072820
• Suggestions for drawing, illustrating, presenting content in creative ways
  • Take similar photos through a non polarized lens and a polarized one, compare and contrast the photos.
• Engineering and design challenges connected to the content
  • Fishermen use polarized glasses to help them see into the water while fishing. How could you use polarized lenses to solve another type of problem (health, environmental, etc.)