Sunday, August 23, 2009

Rainbows...I Have Got to Get a Polarizing Filter!

I was outside as a storm was rolling in and saw a nice rainbow.

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I was looking at it with my sunglasses on...I have polarized sunglasses. Rainbows are polarized so you get a very different view of them if you rotate your sunglasses 90 degrees. I should get a polarizing filter for my camera, but I don't have one so I just held up my sunglasses in front of the camera.

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So you can see the rainbow nicely. I rotated the sunglasses 90 degrees and snapped the next photo mere seconds later (no visible changes took place in the rainbow if you looked at it with your eyes).

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Note how the rainbow almost totally disappears! What's up here?

Okay, rainbows always appear opposite the Sun (this is looking east as the Sun was in the west). Blue light slows down more than red light when it passes into the raindrop and bends more so the blue and red light (and all the other colors) get separated. This process is called dispersion. The light then reflects off the back of the raindrop and heads toward you (and undergoes a second round of dispersion when it leaves the raindrop). The angle of reflection necessary to see a rainbow is about 42 degrees.

When light reflects off of a surface, it becomes polarized. If the angle is right, it can become almost completely polarized. For rainbows, the polarization is about 94% for the primary rainbow (and 90% for the secondary rainbow, when visible). By turning my polarizing filter so it is exactly perpendicular to the angle of polarization of the rainbow, I can block almost all the light at least all that my camera can detect!)

Next time I am at a photo shop, I am buying a polarizing filter for my camera.

Reprinted with permission from the Half-Astrophysicist Blog.

8 comments:

  1. Ok hale... I'm going to have to read this blog several times if I want to get it. :s

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  2. Cool, very cool. Hale, you are so smart!!! But I think I got it...well, I think I do...

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  3. I'm not quite sure either. Hey Hale, guess what we did last night? Gary is a space junkie and last night we took a tour of Mars and the Moon thanks to Google. It is truly amazing what is out there.

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  4. I didn't know that. Hale, you are a fount of knowledge. Any way to set up my Google searches so that they also tap your brain?

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  5. I saw a TV show on 'I am Joe's Brain' or something like that. How different parts of the brain work and when they work AND when they shut down. REAL interesting. BTW I just bought a pair of HD sun glasses and they are awesome. My Blueblockers do block colors and the yellow line in the road looks orange. Greens are real green. Funny.

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  6. Well, if you don't get everything, that's okay. I just gave a VERY broad overview of what is going on here. I would really need to get into refraction of light, a full explanation of dispersion, what is polarization of light and how reflection can polarize light to really give the full picture here. That is a few weeks in a good high school physics course!

    I recommend wearing polarized sunglasses. You can try this yourself next time you see a rainbow.

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  7. The hell with the rainbow, I want the pot of gold at the end of them!

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