Why is the sky blue?

On a bright and clear sunny day, most of us glance at the “Blue” sky spread out through vast distances. During the day we notice that the sky is blue and a reddish yellow haze colour as the sun is about to set. But where does the sky get its colour from, if the whole space surrounding the earth is completely dark and our light source, the “Sun”, gives out white light? Well, this is a phenomenon related to optics and the properties of certain compounds to absorb and radiate light. To understand why and how it happens, let us first look at the properties of light and how it behaves when obstacles are placed in its path.


Why is the sky blue

Why is the sky blue? (Image via: sxc.hu)


Properties of Light

Light is a form of energy that is transmitted in the form of waves containing mutually perpendicular magnetic and electric fields (electromagnetic waves) and travel in straight lines at a speed of 300 million metres per second. Although there are many types of electromagnetic waves depending on the wavelength, light falls in the range of wavelengths which are visible to us.


The white light that is emitted from the sun or that which we can see from a light bulb is actually a combination of many colours of light. These colours can be segregated when white light is passed through a prism. The red light falls on one and violet on the other end. If we arrange the colours with respect to their wave lengths in ascending order, then the visible light range starts with violet and ends with red in the order Violet, Indigo, Blue, Green, Yellow, Orange and Red. Read how is rainbow formed?


What Makes the Sky Blue? Obstruction of Light by Atmosphere

The earth’s atmosphere is composed of gases (78% Nitrogen, 21% oxygen and the rest being inert gases and water vapour), it also contains a small amount fine dust particles. So, the light that reaches the earth from the sun in straight lines is finally obstructed these gas and dust molecules. In this step two cases may occur: One is that when the light is obstructed by dust particles, which are comparatively large when compared to the wavelength of light, it is reflected and scattered into different directions. In the second case, gas particles are much smaller than the wavelength of light and hence some of the light gets absorbed by the gas molecules.  The extent to which light is absorbed depends on how small the difference is between the gas molecule and the light wavelength. Therefore violet, blue, indigo get absorbed to a greater extent. The colour of light which is absorbed is later radiated out. This phenomenon is known as “Rayleigh scattering”.


Light Scattering

Light scattering relative to the position of the sun

For this reason when you look at the sky, you see blue everywhere, since the blue light is absorbed by gas molecules and radiated out.  That’s why the sky is blue. But when you look at the sun, it looks white (or yellow) as usual. This is because; the longer wavelengths directly pass through the atmosphere and appear white (with hints of red, orange and yellow which have a minor effect). As we look forward towards the horizon, the blue colour becomes paler since the scattered light has to pass through more air to reach us.


Evening Red

The colour starts from yellow on the top and gradually progresses to red at the bottom

As sunset approaches, the sun goes further away from sky and appears less bright than during the day. Since it is at a high incidence angle to the sky, there is more reflection and scattering. The sun now falls in front of us rather than overhead during the day. Since there is more absorption and scattering the blue and green light are removed from the white light. Therefore, only the yellow, orange and red pass through the atmosphere. As the angle of incidence increases (sun sets) the colour approaches closer to red. Finally, we observe the true colour of the space outside us, which is eternal black. The colour of space is black as it has no atmosphere to disturb the path of light. The colour of light which is absorbed depends on the size of the particles present in the light’s path. Suppose you go to the moon, you will be surrounded with black whether the sun is facing the moon or not. This happens because the moon does not have its own atmosphere.

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