Why the sky is blue and the sunset red?

Why the sky is blue and the sunset red_
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To answer the question: “Why the sky is blue and the sunset red?” we need to introduce a concept of the wave length.

But, if you can’t wait for the answer, to make the long story short. It’s all because of the light wavelength that is filtered out and scattered by the oxygen and nitrogen molecules.

What is light?

We can go deep into the nature of this phenomenon, but for the purpose of this post, let’s assume that light is an electromagnetic wave. As every wave, it is characterized by its frequency, wavelength, and amplitude.

Wave characteristics
What is the definition of wavelength?

Wavelength is the distance from one crest to another, or from one trough to another, of a wave (which may be an electromagnetic wave, a sound wave, or any other wave).

What is the definition of amplitude?

The amplitude of a wave is the distance from the center line (or the still position) to the top of the crest of to the bottom of a trough. Amplitude is measured in meters.

What is the definition of frequency?

The rate at which a vibration occurs that constitutes a wave, either in a material (as in sound waves), or in an electromagnetic field (as in radio waves and light), usually measured per second (Hz, hertz)

As we have established a light definition, let’s break it down further into the electromagnetic spectrum where we mark the “visible” part.

Source: Wikipedia.org

As you might see, only the small part of the wavelength is visible to us.

Why do we see colors and objects?

The point is that you can see different colors of materials as long as the light is “bounced” from them.

To be more precise, a photon hits the material and once it meets the electron in the given material, it gives it a portion of energy. As a result, this electron is moved into a higher orbit. If it cannot stay in this state forever, it quickly comes back into its previous orbit and emits a photon that comes to your eye with a particular frequency, hence the wavelength. Dependently on the emitted energy, you see different wavelengths and as a result, you see a different color.

The more energy the more blueish the color is until it becomes invisible to us and turns into UV and later on into radiations.

The less energy the more reddish the color is until it becomes invisible to us and turns into the infrared. What is funny, you cannot see it but you can feel it (the heat you feel, is infrared light, so your skin sensors receive it).

The material that we see works as a filter of the whole light spectrum. It absorbs only the particular wavelength that is remitted by the excited electron when lowering the orbit.

Why do we see objects?

Why the sky is blue and sunsets are reddish?

Adding up all the things we’ve learned from the previous chapters, you can probably answer the question yourself.

Let me bring you all the things together.

First, our atmosphere consists mostly of nitrogen and oxygen molecules. They absorb very energetic radiation (UV and above), hence we are safe on our planet.

Secondly, the molecules scatter the light (just like in the prism). The white light from the sun is a mixture of all colors of the rainbow. During the sunset, the blue waves are scattered more than the red wave, so we see the sky more reddish. In opposite, when the Sun is high more red waves are scattered and the blue stays more visible.

What is the Tyndall effect?

The first steps towards correctly explaining the color of the sky were taken by John Tyndall in 1859.  He discovered that when light passes through a clear fluid holding small particles in suspension, the shorter blue wavelengths are scattered more strongly than the red.  Try to pour some milk into the glass of water and light it up from behind.

This is most correctly called the Tyndall effect, but it is more commonly known to physicists as Rayleigh scattering. He showed that the amount of light scattered is inversely proportional to the fourth power of wavelength for sufficiently small particles.  It follows that blue light is scattered more than red light by a factor of (700/400)4 ~= 10.

The summary

If you don’t want to read the whole article, these are the takeaways.

  • The light is an electromagnetic wave that is represented by the frequency, amplitude, and length.
  • All materials absorb the particular electromagnetic wavelength and reemit the particular wavelength.
  • All materials scatter the electromagnetic wave, and as a result, the scattered wave comes to our eyes changed in length
  • Our sky contains nitrogen and oxygen molecules which both absorb and scatter particular light wavelengths and the final visual effect depends on the point of our observation. The more the Sun is over our heads the blueish the sky. The more the Sun is closer to the horizon, the reddish the sky is.

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