Sky appears Blue during daytime, but why

We all know the sky appears blue. But have you ever wondered what causes it to appear bule.

The answer lies in what we call the Scattering of Light. Let’s have a look at how this works. First let’s be clear that the sunlight. which we think is white light, is not really white in its composition. It is made up of seven colours of rainbow.

Now, let’s see what our earth’s atmosphere contains. Well, it has a lot of gasses and other particles floating around freely. And when the sunlight enters, it strikes with the gasses and other suspended particles. This is where the party begins. The atoms and molecules of the gasses absorb the light and re-emit it in all directions.

Let’s have a look at a little experiment with a glass prism. When the white light is made to pass through a prism, it scatters the light and what we see is a pattern of colours of rainbow on the screen.

But the question arises, if there are seven colours in the white light, why do we then see the sky blue and not some other colour like orange or red? We’re sure you must have had the same question. And to answer this, we need to first understand the concept of wavelength. But why wavelength? Because, like other forms of energy, light also travels in waves. Now, some lights travel in short waves while other lights in larger ones.

What is Wavelength of Light

It is the actual distance between the successive crest and troughs of a wave. In simple words, the distance between either the crest or trough of one wave and the next wave is called the wavelength.

When it’s about the spectrum of the white light, every colour has a different wavelength, and they disperse accordingly. It ranges between 400 to 700 nm. And according to Rayleigh’s scattering phenomenon, the shorter wavelength is scattered more than the longer wavelengths.

In the diagram below, you will see that the red coloured light disperses less than the orange one, the orange less than yellow, and so on and so forth. And as you move further, you would observe that the violet has the shortest wavelength, and thus, it scatters the most.

Another question that now strikes the mind is; if the violet has the shortest wavelength and it scatters the most, then why does the sky appear blue, and not violet.

Why is the sky not violet?

To understand this, you need to be aware of these two concepts.

1. Our eyes are naturally inclined to see blue. Yes, that’s correct. The cone cells in our eyes have greater sensitivity towards the blue colour, hence, the sky appears blue to us. Another aspect here being the frequency of the colours. Violet has a greater frequency than blue, and our eyes are quite not sensitive toward violet.

2. The amount of violet in the white light is not as much as the amount of blue. It is to be understood that even though the wavelenegth of violet is lesser than that of blue, it’s the quantity that also matters.

Now, let’s come back to the root question of why is the sky blue during daytime. And the answer to the question is;

a. scattering of light according to the wavelength- shorter the wavelength, more it scatters.

b. sensitivity of our eyes toward different colours

c. and finally the amount of colour present in the visible light spectrum.

And it’s the blue colour which is most abundant in the spectrum and it has lower wavelength than red, orange, yellow and green and our eyes are more sensitive to the blue colour. Hence, the sky appears blue in colour during the daytime.