Γιατί είναι μπλε ο ουρανός

 

Αναρωτήθηκες ποτέ γιατί τα βλέπουμε όλα μπλε (no pun intended) εκεί ψηλά; Η απορία αυτή βρίσκεται κρυμμένη κάπου εκεί στα άδυτα των "γιατί" που ποτέ δε ρωτήσαμε τους γονείς μας ως παιδιά. Που και να το κάναμε, ίσως να μην παίρναμε σωστή απάντηση, καθώς πρόκειται για ένα φαινόμενο που μόνο η επιστήμη μπορεί να εξηγήσει εμπεριστατωμένα. 

When we leave childhood behind, we often forget the curious spirit that characterizes girls and boys. How many times do we wonder why the world around us? Do we know how to answer a priori questions as basic as why the grass is green or why the sky is blue?

There are thousands of unanswered questions , but many others do. In this article we are going to break down a question that we all take for granted every morning when we look out the window: why is the sky blue and not green, yellow or purple? We invite you to bring out your curious spirit and let yourself be carried away by the amazing physical phenomenon behind this question.

Γιατί μπλε;

To answer the question of why the sky is blue , we must first understand what color is.

Colors are nothing more than wavelengths; or, rather, it is what our brain interprets when it perceives a certain wavelength through our organs of vision, the eyes.

Carl Sagan said in Cosmos that "color is to light what pitch is to sound." Light, also like sound, is a wave; and that wave breaks down into the visible light spectrum , which has different lengths. Each of them correspond to the seven colors of the rainbow. This can be seen naturally if we scatter light onto a prism (remember Pink Floyd 's The Dark Side of the Moon album cover ) or when the rainbow itself occurs when it rains.

Each wavelength propagates generating a periodic disturbance; this disturbance is different for each of the colors. Well, the wavelength of the color blue is short and more compressed than, for example, that of the color orange, which is more extended as can be seen in the image below. This is key to understanding what comes next.

Sunlight takes a full eight minutes to reach us, from the moment it is emitted from its surface . (This means that if the Sun suddenly went out, it would take us eight minutes to find out.) As light travels through the atmosphere, which is made up of gases such as nitrogen or oxygen. The key is how these gases interact with the light that arrives from the Sun.

At noon, when the Sun is high, light strikes the atmosphere at almost vertical angles. This makes nitrogen and oxygen tend to scatter the shorter, compressed wavelength light spectrum more easily. Remember? Precisely what we interpret as the color blue, and also the color violet. These waves are scattered by gases in the atmosphere , saturating the sky in blue (and not violet, because our eyes perceive blue better than violet).

This is why we see the sky as light blue on a clear day. So why is the sky orange or reddish at dawn or dusk?

Γιατί αλλάζει χρώμα στην ανατολή και το ηλιοβασίλεμα

That the Sun rises red is not because "blood has been shed tonight" (Legolas dixit ). A few lines back we explained how the sky appears blue in broad daylight, when the Sun's rays hit vertically, from above. But as the Sun 'hides' behind the horizon, the light reaches the atmosphere at an angle more and more parallel to the Earth. That is, light travels a greater distance in the atmosphere to reach us, so more nitrogen and oxygen molecules scatter the light.

Thus, at sunset , just like at sunrise, the atmosphere filters more short waves and tends to scatter longer wavelengths , such as those that correspond to the color orange.

Other factors, such as dust particles or humidity, also come into play in the color of the sky. Therefore, seeing the sky as a pure blue color is a good sign, because it indicates that little pollution is present.

Χωρίς την ατμόσφαιρα ο ουρανός δε θα είχε χρώμα

Light is a wave that travels through space in a straight line without being scattered, unless something crosses its path. In this case, it is the atmosphere that causes the light to decompose. If there were no atmosphere, we wouldn't be able to see any colors , since there would be no gases to scatter light (although, on the other hand, we wouldn't be alive, since we couldn't breathe). For this reason, the sky looks black from the Moon or from any other celestial body that lacks an atmosphere.

 

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