Q: Why (on a clear day) is the sky blue?
A: The sky is blue because of a process known as Rayleigh scattering.
According to classical physics, an accelerated charge emits electromagnetic
radiation. Conversely, electromagnetic radiation may interact with charged
particles causing them to oscillate. An oscillating charge is continually
being accelerated and hence will re-emit radiation. We say that it becomes a
secondary source of radiation. This effect is known as the scattering of the
incident radiation.
The atmosphere is, of course, composed of various gases that together form
air. We may treat each air molecule as an electron oscillator. The electron
charge distribution of each molecule presents a scattering cross section to
the incident radiation. This is essentially an area upon which the incident
radiation must fall for scattering to occur. The amount of scattered radiation
will depend upon the magnitude of this cross section. In Rayleigh scattering
the cross section is proportional to the fourth power of the frequency of the
incident radiation. Sunlight is composed of various visible frequencies
ranging from low frequency (red) to higher frequency (blue) light. Because it
is of a higher frequency than other visible components, the blue part of the
Sun鈥檚 spectrum will be scattered more strongly. It is this scattered light
that we see and so the sky appears to be blue.
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Incidentally we are also able to explain why sunsets are red. When the Sun
is close to the horizon its light must travel through more atmosphere. The
blue light will be scattered strongly whereas red light, because it is of
lower frequency is less prone to scattering and so is able to travel straight
to the observer.