Every day, whenever we look at the sky, we see a blue sky mostly. But why is the sky blue? Can't it be some other colors like green, white, or purple? Well, there are some specific reasons behind seeing this color.
Light is an electromagnetic wave. Though light is the fastest moving thing in the universe, its speed isn't the same in all mediums. All light travels in a straight line unless something gets in the way. But light always uses the shortest and fastest path to go forward, because of which we experience refraction. Refraction defines the change in direction of a wave passing from one medium to another caused by its change in speed. This happens when the medium changes. I ain't going further into this topic. The highlighted point here is light changes its path or direction according to its speed.
The light coming from the sun is white. This is why the sun is seen as white in the morning. But we know that white light is the combination of all seven rainbow colors. We can separate it through a prism. This color difference happens because of different wavelengths. Blue light waves are shorter than any other visible light wave. Blue lights have wavelengths between about 450 and 495 nanometers and red lights whose waves are longer than any other visible light wave, have wavelengths of around 620 to 750 nanometers.
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| Prism |
When sunlight reaches the earth's atmosphere, it scatters in all directions because of all the gases and particles present in the air. But it scatters according to their wavelengths. It will scatter more if the wavelength is less and the reason for scattering is the refraction of light that we discussed before. As a result, blue light is scattered more than other colors. That's why we witness blue sky most of the time.
But in the dawn, the sky fades to lighter blue or white color. Why does this happen? At that time, the sunlight comes from low in the sky and it has to pass through more air than it needs to reach us from overhead. And because of this more air levels, the blue light scatters and re-scatters many times in the sky and goes in many directions. Besides the air levels, the earth's surface also reflects light. This scattering mixes the colors together and we know that the mixture of colors brings white light. As a result, we see more white than blue and the sky looks lighter blue.
But do we experience some other color when the sunlight is available? Think about the evening. In the evening, the sky looks more reddish than bluish. But what makes this red sky or sun? As the Sun gets lower, its light requires passing through more atmosphere to reach us. However, this time the blue lights scatter the most also but because of its less wavelength, it can't reach to us fast. This allows the red and yellow light to come straighter and faster to our eyes than the blue light because of their longer wavelength. As a result, we see the red sky and red sun in the evening.
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| Red Sky |
Written by: Tasnim Ferdous
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