Scattering/Polarization - Blue Sky



Description of the Process Used to Take the Photographs


This photograph was taken on Wednesday, May 28 at 4:00 pm. This time was
chosen because the sun was not particularly bright and the sky
was an ideal shade of blue. No other process went into this photograph.


Physical Concepts and Principles Involved in the Image


This photograph illustrates a blue sky which is a physical phenomena. After all, when light is
emitted from the sun it is a mixture of many of different colors with different
wavelengths. Why, then, does the sky appear blue?
The answer to this lies in the phenomena of scattering and polarization.

In the mid nineteenth century, it was discovered that when light passes through a
fluid containing small particles, the shorter wavelengths are scattered (change direction)
more than larger wavelengths. Thus, it can be concluded that because blue light has one
of the shortest wavelengths (in the visible spectrum), it is scattered the most.

Just as light can be scattered in a liquid, it can also be scattered in the Earth's
atmosphere. The sky appears blue because molecules of nitrogen and oxygen in our
atmosphere scatter this wavelength the most.

When waves of light are scattered, they also become polarized because their paths
change by different amounts depending on their wavelengths. This polarization is most
obvious when looking at the sky while wearing polarized sunglasses; the sky will usually
appear a deep blue.

Historical Background of This Phenomenon


In 1859, John Tyndall first conducted the experiment that determined that when
light passes through a clear fluid containing small particles, different wavelengths are
scattered by different amounts.

A couple years later, Lord Rayleigh began to study this experiment and
determined that "the amount of light scattered is inversely proportional to the fourth
power of wavelength for sufficiently small particles". Using this equation, he discovered
that blue light was scattered approximately ten times more than red light.

Finally, in 1911 it was Einstein who applied the theory of scattering to the Earth's
atmosphere and the color of the sky. Likewise, he also created several scattering
equations.