ISM, Page 63: [Previous Page] [Next Page] [Class Home Page] Textbook pages 368 - 374

Oh! So, THAT's why the Sky is Blue!

Indeed, you should have noticed that the description of why Reflection Nebula are blue-ish in color can be extrapolated to explain why our sky is Blue! But here is the full explanation:

LIGHT IN THE AIR
Perfectly reflected light appears white because it still contains all of the same colors. However, gas molecules are smaller than the wavelength of visible light. When light hits a gas molecule, some of it may get absorbed. After awhile, the molecule radiates (releases, or gives off) the light in a different direction. All of the colors can be absorbed, but the higher frequencies (blues) are absorbed more often than the lower frequencies (reds). This process is called Rayleigh scattering.

WHY IS THE SKY BLUE?
T he blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, little of the red, orange and yellow light is affected by the air. However, much of the shorter wavelength light is absorbed by the gas molecules then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue.


WHY IS THE SUNSET RED?
As the sun begins to set, the light must travel farther through the atmosphere before it gets to you. More of the light is reflected and scattered. As less reaches you directly, the sun appears less bright. The color of the sun itself appears to change, first to orange and then to red. This is because even more of the short wavelength blues and greens are now scattered. Only the longer wavelengths are left in the direct beam that reaches your eyes.