Contrary to Popular Belief, Air is Not Colourless; It’s Blue!


The Sunday Nation


03 July 2005


Why is the sky blue? If there are stars all over the space, where do they disappear to during the day? Why does the sun appear reddish in the morning and in the evening and white at midday? All theses questions are related, but let us see how.

The quick answer to why the sky is blue is that air is blue in colour! That may be surprising because we all “know” that air is colourless. The reason why it appears colourless is that we normally look through very short distances (a few kilometres) of the atmosphere.

When we look at larger distances (say, tens of kilometres in the sky), we notice the faint blue shade. In the same way, clear glass is actually green. If you look through the narrow side of a glass pane, you will see the green colour.

The question why air is blue may appear futile: it sounds like asking why charcoal is black – of course God made it that way! However, we can try to answer it. First, we note that sunlight is made up of many different colours ranging from red to violet (which is a deep purple, or blue).

When the light travels through the atmosphere, it is scattered in all directions by the air molecules. This scattering is much more pronounced for the bluish colours of light than the reds.

So, when a beam of sunlight is moving through the air, its blue components are constantly scattered in a wide area while the other colours remain in the straight-line path. From the ground, this scattering is seen as the blue of the sky. Indeed, if you observe carefully, you will notice that regions of the sky that are further from the sun have a deeper shade of blue.

That also explains why no stars visible during the day. The intensity of the scattered blue light is so right that it outshines the light from the stars. At night, there is no sunlight so we are ale to see the stars. If there was no atmosphere, the daytime sky would be dark and stars would be visible all the time.

When the sun is near the horizon, its rays travel through a much larger distance through the air than when it is overhead. Although the Earth’s atmosphere extends to about 100km above the ground, when you look horizontally, you see through more than 1,000km. This is due to the curvature of the planet. Remember: the atmosphere has the same shape as the Earth!

Now, the greater the distance that light travels through air, the more blue-scattering it will undergo. The scattered blue light is subtracted from the main beam leaving only the reddish colours. Thus the sun appears red.

In addition, since a large portion of the sunlight is removed, the intensity is also reduced significantly. That is why we are able to look at the sun directly while enjoying the sunset (or sunrise).

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