Hobie Science: Surf Science 101: Why is the Sky Blue?

For this weeks science blog we will leave the oceans and look at what makes the sky appear blue. And no, it is not because the ocean is reflecting the color of the water into the sky. (Ed Note: Full disclosure, I have always thought this to be true before Gary explained it to me)

In order to understand what makes the sky blue we must first understand the basics of sunlight and color. I recently read a fascinating article about a young Isaac Newton’s early experiments with light waves. The year was 1665 and a 23 year-old Newton was home from school due to the plague. Newton confined himself to his room and began experimenting with a prism. He closed his blinds and cut a small hole allowing a small stream of light to pass through. Newton then held a prism into the beam of light causing the light to scatter, ultimately casting a rainbow upon his wall.

“Spectrum of Time” by Peter Erskine

Prior to this experiment it was widely believed that sunlight was white in color and therefore could not be muddied or changed. What Newton proved was that sunlight actually consisted of all the colors in the visible light spectrum together. There were many people who criticized this experiment, mostly due to religious credence, claiming that white light was holy in nature and could not possibly be made up of colors, and that the prism itself contained the colors of the rainbow.

Subsequently Newton devised a second experiment consisting of a second prism. Remember, this was in 1665 and a prism was not easy to come by. So, Newton held the second prism into the blue portion of the rainbow allowing only blue light to pass through and just as he had presumed, only blue light was cast against the wall.

Visible light is part of the electromagnetic spectrum consisting of the electromagnetic wavelengths between about 400 and 800 nanometer. For reference, one-nanometer is one-billionth of a meter.

As we can see from the electromagnetic spectrum above, the colors containing the highest amounts of energy, thus the shorter wavelengths, are violet and blue. When sunlight passes through the atmosphere these higher energy wavelengths scatter, being absorbed and reflected off the molecules that makeup our atmosphere causing the sky to appear blue. In space, where there is no atmosphere, the light is not being scattered off of molecules therefore appearing black.

Why, then, does the sky not appear violet? This has to do with how our eyes work. We have three different light receptors in our eyes called cones. They respond most strongly to the colors blue, red, and green. Therefore, although violet and blue wavelengths are being scattered in the atmosphere our sensory system, using the blue cones in our eyes, tell our brain that the sky  is blue as it is the strongest wavelength stimulating the blue cones.

So this week when you are at the beach, looking up at the blue sky, you will have a basic understanding of why you see blue, but stick around until sunset and the orange and red sky will open up a whole new set of questions. This will be discussed next week.

Gary Larson


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