Shine a light through polarized sunglasses and the brightness decreases. If you hold two pairs of sunglasses one way, the light then is completely blocked! Not only that, but when you insert a third pair in between the two allows light to pass through again! Spooky!
Materials
- Three pairs of polarized sunglasses (or three lenses from two old pairs)
- Sunny window
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- First, hold up one polarizer up between your eyes and the window. Notice how dim just one lens makes the incoming light from the window to your eye.
- Rotate that polarizer 90o. Does the light intensity (brightness) change? (It should not.)
- Now stack a second polarizer in front of the first, so you’re looking through two polarizers to get to the window. (The image above shows a computer screen with two polarizers on the screen. The polarizers are in alignment with each other so that some light gets through.)
- Rotate one of the polarizers 90o so that the light is completely blocked. You should not be able to see the window at all through the polarizers. The image at the right shows the top polarizer rotated 90o and blocks all the light from the computer screen.
- Repeat steps 3 and 4 and play with it a bit, until you’re comfortable with the steps, then move onto the next step.
Imagine a picket fence—the kind with spaces between the wood. The polarizers are like picket fences in that they block out light that is in a different direction. When you rotate one of the polarizers so that it’s 90o from the first one, it’s like rotating one picket fence 90o so that there are now very few gaps for light to get through.
Make sense so far? Now let’s add the last piece:
Insert a third polarizer in between the first two at a 45o angle to each of them, like in the image at the right. What do you see happen?
What’s Going On?
The secret to making sense of this mystery is taking a look at one polarizer at a time.
Imagine having a polarizer that has it’s lines running vertically, like a picket fence. Any light that is also vertical will be able to pass through.
Sunlight is unpolarized, meaning that it’s in all directions. When it hits the first polarizer, only light that has components in the up-down vertical direction may pass through.
So if the incoming light is all completely vertical, then all the light will pass through and not lose any brightness at all.
If all the incoming light is horizontal, then none of it will pass through, since it’s all blocked. What if the light is at a 45o angle?
Well, some of the light passes through and the rest does not, since light in this orientation has both vertical and horizontal components. Only the vertical component of the light is allowed to pass through the first filter, which in our example is about 71% of the light may pass through.
When that light hits the second filter, which is at a 45o angle from the first polarizer, again some of it is allowed to pass through and the rest is blocked.
The same is true when the light hits the third polarizer. Some passes through and the rest is blocked. When you total it up, about 25% of the original incoming light passes through all three filters.
When one polarizer is at a 90o angle from the second, then all the light is blocked, because none of the light coming out of the first polarizer has any components that are aligned with the second polarizer.
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how do you watch the experiment videos?
Great, got it now, thank you very much.
Did you watch the video the entire way through? It’s because the amount of light that gets blocked depends on the angle between two polarizers. When you only have two, then it’s a 90 degree angle, which wipes out all components of light (vertical and horizontal). If there’s a third at a 45 degree angle inserted, then the light goes from one polarizer to the second, but there’s only a 45 degree angle difference, which means it does NOT wipe out all the light, but still allows some to pass through. Then it does it again by going through the second and third polarizers, since those are again only 45 degrees, not 90, from each other. You must look at the angle one polarizer is to the next one in order to figure out how much light is allowed to pass through, not the initial and final polarizer angles.
Help, we still don’t understand why when two polarisers are at 90 degrees to each other a third one in the middle at 45 degrees let light through. the two block out just about all the light but the middle one at 45 degrees changes the light let through the 1st one so it go through at a different direction???