physics question #3476



kaushik, a 16 year old male from bangalore asks on June 4, 2006,

Q:

Why is the sky blue?

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the answer

Barry Shell answered on July 21, 2006, A:

The sky appears blue due to a phenomenon called Rayleigh Scattering. Sunlight is made of many colours--the colours of the rainbow. The difference in colour is caused by light with different amounts of energy. Each colour has a different energy with the blue end being more energetic than the red end of the spectrum (rainbow).

Air is made of mostly nitrogen and oxygen molecules with tiny particles of dust as well. When sunlight enters the atmosphere and hits these molecules the light is scattered in different amounts depending on its colour or energy. Blue light is scattered the most, and yellow and red light the least. Because of this scattering, the entire sky glows blue, but the sun itself looks yellow.

This also explains why the sky looks more blue straight up, and pale blue towards the horizon. It's because you are looking through more air when you look toward the horizon. This means that the already scattered light must pass through much more air to reach your eyes and it gets scattered even more, so it loses more of its colour tending to pale blue or even white.

Rayleigh scattering also explains why sunsets are red. As the sun gets lower in the sky, the light from it must pass through a greater amount of air, and the scattering of blue light is increased. This makes for a very deep blue sky with a very yellow or red sun as it reaches the horizon. Only the red light comes directly to our eyes, while all the blue light is scattered away by the air molecules. Wide ranging redness on the horizon is caused by dust particles scattering the light in a somewhat different way. A very good explanation of this can be found at Science Made Simple by Roberta Cahn.

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