Don't worry, color isn't ending. As far as I know, you'll continue to see the world in all of its vibrant hues. Well, maybe not you. We'll talk.
Anyways, this is our last post into the origins of color. I had a ton of fun with all of the researching as well as the writing. Hopefully you all got something out of it as well. Now when a small child ever asks you why the sky is blue or why leaves are green you can give them an answer that ensures not only their scientific literacy, but that you'll never be asked stupid questions again. That, my friend, is a win-win.
This last week's post deals with colors that come about from various optical phenomena, namely scattering and interference. In a nutshell, whenever light hits something that prevents it from traveling in a straight line, the light has been scattered. If the light is stopped completely, we say that the light was absorbed, which is just a very extreme case of scattering. In some cases, the light is scattered in such a way as to send all parts of the spectrum back to your eye equally. Those items appear to be white. In other cases, the scattering is not equal, and the unequal scattering results in some parts of the spectrum making it to your eye, and some parts not. This brings us to our friend, the blue sky.
The sky is blue due to a form of scattering called Rayleigh scattering. Rayleigh scattering, named for Lord Rayleigh, he of the argon discovery, is scattering done by particles that are smaller than the wavelength of light. I'll spare you the formula, but the scattering is dependent upon the size of the particles and the wavelength of light. In fact, the intensity of the scattered light is inversely proportional to the wavelength of light, which means that when one thing (the wavelength) goes down in value, the other thing (the amount of scattering) goes up. Not only is the relationship an inverse proportional one, but the intensity is inversely proportional to the wavelength to the power of 4. In other words, really small wavelengths, like, oh, I don't know, the blue part of the spectrum, have a high intensity when scattered, while light with a large wavelength, like red, has a low intensity. This is why the sky seems uniformly blue, the intensity of the blue scattered light is incredibly high, and it appears to be coming from everywhere. Light comes from the sun, hits the particles in the atmosphere, gets scattered and voila, the sky is blue.
During sunrise or sunset, the light you're seeing has to travel through a much larger chunk of the atmosphere to get to your eye. During its travels, the light has the blue parts of the spectrum scattered multiple times and essentially stripped out. What you see is all that's left, namely violet and red. OK, that last part isn't true. The blue parts aren't stripped out, they're just being scattered to the ground earlier in the light's journey. Just because it's sunset for you, doesn't mean it's sunset for everyone there pal. While you're looking at the red light from scattering, someone else is seeing the blue portion. Somewhere in there is some lesson about togetherness or some such bullshit. FYI, the particles in clouds are too big to scatter the light via Rayleigh scattering, so the light gets scattered uniformly and the clouds appear white.
Interference is what happens when two or more waves intefere with each other so that a new wave is created in the process. It usually has to do with waves that are coherent with each other, because they have the same source or the same frequency. Let us not forget that light is a wave, and as such, it can be interfered with. When light passes through a plane with multiple slits in it, the crests of some of the waves interact with the troughs of some of the other waves and the two cancel each other out. As a result, bye-bye light. The size of the slits determines which frequencies of a wave are amplified. This becomes important later.
In the case of butterflies, such as the beautiful Blue Morpho and birds, such as the peacock, the color of the wings has to do with interference. A butterfly's wings is covered in very small scales, that are layered in such a way as to provide multiple slit interference. Ditto for peacock feathers, only instead of scales, it's thin, plate-like layers, like, you know, scales. Light that hits these scales travels through the slit, and reflects off the scales at the same time. The resulting new wave combination amplifies the blue part of the spectrum for the Blue Morpho, and every goddamn part of the spectrum for the peacock, depending on what you're looking at. The various colors of the peacock are caused by the size of the slits as well as the thickness of the layers of plates and the angle of view. Interference is also the cause of color for pearls and beetles. Who knew that something so simple as two light waves smashing into each other could be responsible for such beauty. Truly we could all learn something from violence. Wait, no, ah, never mind.
Next week we tackle the first in a long line of reader requested questions as we find out why that Pikachu tattoo looks so damn good now, but will only serve to annoy and terrify you with it's blurry, indistingushable features when you're 75. 'Til then Squaddies, take care.
Wikipedia - Interference
Wikipedia - Iridescence
Wikipedia - Rayleigh Scattering
WebExhibits - Causes of Color