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

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

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.

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).

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.