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The Question
(Submitted June 11, 1997)
Why is the sky blue and why is the Sun red at sunrise and sunset (taking
into account the properties of interstellar dust)?
The Answer
Actually, it is not interstellar dust that is responsible for
the color of the sky during the day and the Sun in morning and evening.
It is the scattering within the Earth's atmosphere that is
responsible.
Visible light is the region of the electromagnetic spectrum
with wavelengths from 0.00000035 meters (violet light) to 0.00000075 meters
(red light). Light usually travels in a straight line path, but
it can be scattered by particles such as the nitrogen molecules in
the Earth's atmosphere. This means that portions of the light energy
are sent off into different directions. These directions are determined
by the size of the wavelength of the light and the size of the particles
doing the scattering. For the Earth's atmosphere, the effect is strongest
in the shortest wavelengths; the effect is very small at longer wavelengths.
The scattering is dominant for the wavelength that we see as blue, and
this is why the sky appears blue.
When the Sun is close to the horizon, such as at Sunrise
and Sunset, the light must pass through a much longer path in the
atmosphere; more of the blue light is scattered out of this direction,
leaving only the red light. This is why the Sun appears red at
Sunrise and Sunset.
Cheers,
Padi Boyd
for the Ask an Astrophysicist
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