Why is the Sky Blue?

Objective:

To see how the scattering of light can divide light into its different colors, causing the sky to have different colors. For the more advanced student, one can relate this to the different wavelengths of light. Also included: experiments on refraction and reflection.

List of Materials:

• Flashlight
• Laser pointer
• Flour or Chalk dust
• 2 liter clear plastic soda bottle
• Smooth sided wine glass
• Water
• Milk (whole works best)

Procedure 1:

Sift flour or chalk dust over the flashlight beam so that the particles fall through (Scattering) it. See how the light flashes as it hits each particle.

Procedure 2:

Fill the soda bottle with water and add a small amount of milk to the water.

(Scattering) Shake the bottle to mix them up. If you place the flashlight so that is shines through the side you will see blue light being scattered. You may have to keep adding milk until you see this. For a sunset, continue adding more milk until the side of the bottle starts to scatter oranges and then reds.

Procedure 3:

Fill a smooth sided wine glass with water. Add a few drops of WHOLE milk to the

(refraction) glass. With a laser pointer, shine the laser perpendicular to the glass to see the laser light go through the liquid. Now shine the light at an angle to the side of the glass, and observe the laser light from the top of the glass. The light will bend towards the middle of the liquid. This is REFRACTION - the bending of light as it passes from one substance (glass) to another (liquid). It is caused because light travels at different speeds depending on the density of what it is traveling through.

Procedure 4:

Shine the laser pointer at an angle up from the bottom of the wine glass towards the

(reflection) top of the glass. You will see the laser light REFLECT off of the top of the liquid back down towards the bottom of the glass. Specifically, this is an INTERNAL REFLECTION, since the reflection takes place within the water.

Scattering is the dispersal of particles in a variety of directions. When the flashlight shines on the flour, you see white flashes as the light hits the particles. In the atmosphere, it isn't chalk dust or flour that scatters, but the different kinds of molecules that make up the air. Light is made up of all the colors of the rainbow, each with its own wavelength. This is called the electromagnetic spectrum. When the path length of sunlight is the shortest, the middle of the day, the sky is blue, because of shorter wavelengths being scattered. At sunrise and sunset, when the sun has more distance to travel through the atmosphere, the shorter wavelengths are scattered out, leaving the longer oranges and reds.

Refraction is the bending of light as it passes through two materials of different density. As the light passes from the less dense air to the more dense water, it slows down. This is why it is hard to spear a fish or pick up an object in the bottom of an aquarium. The refraction also causes the light to separate into colors. When added with an internal reflection, you have all of the necessary ingredients to make a rainbow!

Wavelengths and the corresponding colors*

 Color Wavelength interval** Typical wavelength** Violet 0.390 - 0.455 0.430 Dark Blue 0.455 - 0.485 0.470 Light Blue 0.485 - 0.505 0.495 Green 0.505 - 0.550 0.530 Yellow Green 0.550 - 0.575 0.560 Yellow 0.575 - 0.585 0.580 Orange 0.585 - 0.620 0.600 Red 0.620 - 0.760 0.640

** Measured in Micrometers (10-6 Meters)
1 micrometer is approximately 0.000039 inches!
White light is composed of all of the colors.

* From Wallace and Hobbes, "Atmospheric Science," Academic Press, New York, 1977.

References:

Blue Sky Math

Extra Credit:

Students can study the Law of Reflection and Law of Refraction to better understand the optics involved in atmospheric phenomenon. See if they can explain how halos, sun dogs, solar pillars and glories are formed using the laws of reflection and refraction.

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