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Sep. 30, 2011

Science Dad on What Makes the Sky Blue

by Vince Harriman

Click to enlarge images

Last week when we were making our flubber and oobleck we noticed something a bit odd when we were looking at our photo-documentation of the experiments. The photos of oobleck and flubber all had a slight blue cast to them. That got us talking about colors in general which led to that classic question -- what makes the sky blue?

Take a look at this picture of flubber, which was made with clear water, clear crystals, and white glue. Where is the blue coming from?

Flubber giving off blue light

It isn't just the color of the counter, because look how much Ian's t-shirt is reflecting orange at the top of the photo. Take a look at this still I captured from the video on last week's post:

Still from video

The material in the bowl isn't as blue as the sky, but look at bluish tinge in the right of the photo. What was going on? Beckett and I decided to investigate a bit further. Remember I described both flubber and oobleck as colloids? A colloid is a substance microscopically dispersed evenly throughout another substance. When light hits a colloid, the shorter wavelengths of light are scattered while many of the longer wavelengths pass through. This happens all the time in daily life -- radio waves pass through walls and trees and metal car roofs, but light waves are reflected.

In most colloids, the suspended particles are just large enough to scatter the shorter visible light waves -- which we see as the blue cast coming off the white. Most white and translucent colloids exhibit this effect, known as the Tyndall Effect. The Tyndall Effect, also known as Tyndall Scattering, was named after John Tyndall, the 19th century physicist who first described it. You can see the Tyndall effect by pouring yourself a glass of milk -- milk is an emulsified colloid composed of liquid butter fat suspended in water. In colloids with a strong color, blue light is scattered, but the color of the suspended particles dominates the faint blue color of the Tyndall effect.

You can also see the Tyndall Effect in a location that might surprise you: blue eyes! Blue eyes are caused when the gene that normally supplies melanin to the eye is suppressed, leaving an iris with little pigmentation. Since the layer of liquid over the iris is an (almost colorless) colloid, the molecules in the liquid absorb longer wavelengths of light and scatter the shorter wavelengths.

Image courtesy Wikimedia commons

When you see blue eyes, you are not seeing blue pigmentation, but a lack of melanin pigmentation and light back-scattered by the Tyndall Effect. So, is this what makes the sky blue? Not exactly, but close.

First of all, the earth's atmosphere is not a colloid but gaseous. Gases are not colloids because they mix and disperse evenly. The 78 percent of atmospheric molecules that are nitrogen gas molecules mix evenly with the 21 percent that are oxygen gas molecules and with the 1 percent that are other gases -- one kind of molecule is not suspended in the other.

But the gas particles in the atmosphere do scatter light particles randomly. This is called the Raleigh Effect or Rayleigh scattering, named after John William Strutt, the 3rd Baron Rayleigh, a near contemporary of John Tyndall. In space, the sun looks like a large white ball -- just like most of the stars we see at night. When the white light of the sun hits the earth's atmosphere, a portion of the light is scattered. Unlike Tyndall scattering (where the molecules suspended in the colloid are large enough to absorb the shorter wavelengths) in Rayleigh scattering, all the wavelengths (both long and short) are scattered in the atmosphere. So what we see, is the average of all of the wavelengths of light molecules bumping around:

Rayleigh scattering varies according to two things: the wavelength of the light and the size of the particles. In our atmosphere, the shorter wavelengths get scattered more, so we see more blue.

You can also see Tyndall Scattering in the atmosphere -- cars (with oil problems) and some motorcycles give off enough particles to form a colloid known as a solid aerosol. If you look directly at the sun (which you should only do with proper eye protection and only for very very brief periods of a second or two) the sun looks white or yellow in the middle of the day, and looks orange or red at night. At the horizon, the sunlight that travels to you passes through much more atmosphere and almost all of the shorter wavelengths are scattered, allowing only the red and orange wavelengths to reach your eyes. Beckett's Aunt Leota (also a science geek) took this picture of sunset this week in New Mexico with the shadow of the Sandia Mountains giving a cool effect:

Rayleigh Scattering at Sunset, New Mexico

In this photo you can see both blue and red light scattered by the Rayleigh Effect. In the center of the photo the Sandia mountains cast a shadow over the landscape, but the light in the upper atmosphere is scattered and blue is still the dominant color. The portion of the photo that shows red is where the sunlight is still traveling through the atmosphere, scattering more, therefore scattering away more of the blue wavelengths and allowing only the red and orange wavelengths to pass.

The final factor that influences the color of the sky is the non-gaseous particles suspended in the atmosphere. Water droplets form clouds and can have a variety of colors. And water droplets at just the right density and angle can form rainbows. Solid matter, from human pollution as well as volcanoes and forest fires, also contributes to the variety of sky colors.

Tell us about your favorite sky color -- summer sunsets or early morning runs, or high noon on the 4th of July. We'd love to hear what makes the sky perfect for you!

About Vince Harriman

Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-6 and Rowan-2 1/2 ask him 'why' approximately 6,542 times a day.

The views expressed are those of the author and are not necessarily those of Science Friday.

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