Perhaps one of the first science questions that pop into anyone's mind is: "Why is the sky blue?"
Indeed, this is one of those questions children often ask their parents or even scientists. (Along with "where do babies come from?")
At this time of year, with crisp clear fall weather, the sky seems a particularly striking shade of blue, albeit darker at the zenith than the horizon.
There are many ways to answer the question of why is the sky blue but recently, when I asked this question several young students responded: "It's not. It's black."
They are right. The sky is actually black.
Air or, more accurately, the gases which make up the atmosphere don't absorb light in the visible region of the spectrum.
They don't absorb or emit any of the colours we associate with the rainbow. They are what we call "optically transparent."
So, when you look up at night, you are really seeing the colour of the sky - that is, there isn't any. As a consequence, at night all you see is the black of outer space which is pretty amazing all by itself.
During the day, though, when the Sun (our closest star) is out, the sky takes on its familiar blue hue and the simple answer to the question is to say it's due to Rayleigh Scattering.
That might appear to explain everything except what is Rayleigh Scattering?
Technically, it is the scattering of light without a change in the wavelength or frequency. But more succinctly, scattering of light is pretty much the same as scattering anything else. It sends light in all directions.
The real question is how and why does this make the sky blue?
If I may draw an analogy, consider walking down a bumpy street. With a long enough stride, you miss out on the bumps. You step over or past them. They do not affect your stride.
More importantly, they would have very little impact on your direction.
Indeed, if the bumps are small enough and your stride is long enough, you might not even notice they were there.
But what happens as you shorten your stride because you are getting smaller? As your stride shrinks, you take more steps and you are much more likely to encounter the bumps.
The smaller the stride, the more bumps you encounter. Indeed, if your stride is small enough, every bump becomes an obstacle in your path.
You might end up having to deviate from your path. If the bumps are large enough or you are small enough, you might just get knocked sideways by a bump, sending you off in an entirely new direction. You might even be knocked backwards.
This is where the analogy stops.
You, as a human being, would be able to correct your course.
After all, you have places to go, people to see, things to do. But the same sort of thing happens when a photon of light encounters a molecule or particulate matter in the atmosphere and photons have no sense of direction or purpose.
They simply travel in a straight line until they hit something where they can be absorbed, reflected or scattered. Scattering can and does occur in every direction.
Each photon of light has associated with it a wavelength or frequency (the two are related) which gives rise to its particular colour, from the short waves of the ultraviolet and blue end of the spectrum through to the long waves of the red and infrared region. A prism or a jar of water is able to break sunlight up into a rainbow based upon all of these different wavelengths.
The sun emits a broad spectrum of light with all wavelengths present. When it enters the atmosphere, it starts to encounter objects - dust particles, aerosols, water droplets and gas molecules.
Each of these objects interacts with the incoming light.
Some encounters result in a scattering of the photons.
The size of the particles in the atmosphere and the distance between them results in the short waves of the blue and violet end of the spectrum being more effectively scattered than the long waves of the red of the spectrum. So it is the blue which gets scattered while the red and yellows pass right on through for the most part.
Since the blue light is sent in every direction, the sky we see looks blue when we look at it.
The sun looks yellow because the blue end of the spectrum has been removed and it is the yellow end which remains.
In the morning or evening, when sunlight has to travel a longer path through the atmosphere, even the yellow light gets scattered and we get the spectacular reds of sunrises and sunsets.
That is why the sky appears to be blue.
