This is Rima, the white antelope again, back for more science.
It’s been about three and a half years now and my obsession has always been this a cappella country band, Home Free (I’m restraining so hard from telling you more about them). Their latest song as of the time of writing this is a cover of Keith Urban’s Blue Ain’t Your Color (not colour because Americans).
After listening to this song for who knows how many times (my guess is a few thousand times), I was inspired to write science posts based on two lyrics from the song. This post is the first of the two (just to make it easier for you to digest).
Blue background: some basic information on the colour blue
Blue is one of the seven colours of the visible spectrum (You can check out my previous post #rainwhoa, where I talked about the visible spectrum and obtaining the seven colours from white light). These colours are described by their wavelengths. Since wavelengths are found in a range (like blue is a range from 450 – 495 nanometers), there is a range of ‘blue’. A greener-blue (or more commonly known as turquoise) would mean that the wavelength is longer, more towards the green side, and a darker blue would mean that its wavelength is shorter, leaning towards the indigo and violet side.
The visible spectrum. Note how each colour is not a solid colour, as their wavelengths are part of a range.
Why is the sky blue?
Inspired by the lyric: Blue looks good on the sky
As sunlight reaches our Earth, it scatters in all directions, thanks to the particles in the atmosphere, like nitrogen and oxygen molecules. Light is then scattered through Rayleigh scattering, which is more effective at shorter wavelengths. Since blue light has short wavelengths, it is scattered more easily by particles in the atmosphere. As you look up, some of these blue scattered light reaches your eyes, so you see the sky as predominantly blue.
Blue sky, white horizon
If you look at the blue sky carefully, you would realise that the gradient of blue gets lighter in the horizon. For the sunlight to reach our eyes when it travels towards the horizon, it has to travel through more particles, scattering more of the blue, leaving with close to no blue light to reach our eyes.
In addition to that, the surface of the Earth reflects and scatters whatever light that reaches it, and this causes the light to mix again, so will see more white and less blue.
Violet ain’t the sky’s colour; Why not a violet sky though?
You may argue that indigo and violet have shorter wavelengths, compared to blue, so technically we should see purple skies according to Rayleigh scattering. That is true, but our eyes are more receptive to blue light and so we register blue light better. Even as our eyes receive a spectrum of blue and violet light, the cones in our eyes perceive it as a mixture of blue and white light.
Also, the strength of the violet light from the sun is not as strong as the blue light. In other words, there is not much violet to be registered anyway.
Red/orange looks good on the sky too!
During sunrises and sunsets, the sky is not really blue, it has a more red/orange tint.
When the sun sets, more of the light will be reflected and scattered as light must travel further though the atmosphere. This means that more of the shorter wavelengths of blue and green are scattered, leaving only the longer wavelengths of red and orange to reach your eyes.
Stay tuned for the second part of this instalment, inspired by another line from the song. Go ahead and make some guesses.
P.S. Aren’t you glad that I did not promote any product/organisation today? Maybe in the next one…