Why is the Sky Blue?
Page 5

1. Light is electromagnetic radiation.
2. The changing electric field of a light wave jiggles electrons as it passes near a molecule in the atmosphere.
3. A moving electron is a moving electric charge, a current of electricity which creates a magnetic field.
4. The electric field-magnetic field combination is electromagnetic radiation, that we say is scattered.
5. All wavelengths of light are scattered but shorter wavelengths are scattered more.
6. Of visible light violet is scattered the most, but because the human eye is more sensitive to blue the sky appears to be blue.

COMPLICATIONS - AND THERE ARE MANY...EVEN MORE THAN LISTED HERE

1. Rayleigh Scattering refers to scattering by molecules much smaller than the wavelength of the incident light. In this situation all electrons essentially experience the same electric field at the same time.
2. When the radius of a molecule (or particle) is about 20 times larger, or more, than the wavelength of the incident light refraction, reflection and diffraction can account for what happens when the particles or molecules are struck by light.
3. When the radius of a molecule or particle is about the same size as the wavelength of incident light the problem is very complicated. With particles of this size the scattering is called Mie Scattering, after G. Mie who, in 1908, first solved the mathematics of the problem in 1908.
4. These pages have covered single scattering, but light undergoes multiple scattering on its way through the atmosphere, an additional complication.
5. As the number of particles, molecules etc increase in the atmospohere and the range of sizes increases, scattering of light becomes more nearly equal in all visible wavelengths, the sky becomes white.
6. Cloud droplets are clear, microscopic water drops, yet a cloud looks white. Cloud drops are large when compared to the wavelength of light, so they scatter all the colors of the visible spectrum. In addition there are so many cloud drops that nearly all the light is scattered many many times so we see white eventhough the individual droplets are clear.

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