Why does the sky look blue during the day and black at night?

Tiny dust particles are floating in the atmosphere around the earth. When light from the sun reaches the earth, it is scattered by those particles and the wavelength of blue light is so short that it is the most diffused, according to Rayleigh's scattering formula. Due to which the sky looks blue in daylight and at night there is no scattering due to the absence of sunlight and so the sky looks black.

Well, what is the colour of sunlight!! Isaac Newton examined and showed that the white light from the sun was actually a combination of seven different colours. The value of the wavelength of light of each different colour (visible) is different. Red light has the longest wavelength and purple light has the shortest wavelength.

In 1859 John Tyndale made an excellent experiment. Mix milk and soap in water. Now when light is thrown on one side of the liquid container, it is seen that the blue light rays of light are scattering more from the other side (Tyndall effect).

Soap and milk particles in clear water create this scattering of light. A few years later, scientist Radley studied the matter in more detail and discovered a new theory of light scattering, known as Rayleigh scattering. According to Rayleigh scattering, the scattering is disproportionate to the wavelength (fourth power).In simple terms, this means that the shorter the wavelength of light, the more scattered it will be and the wavelength of blue light is much shorter than that of red light. Purple light has the shortest wavelength (among visible light).

Now the question is, water vapour floats in the atmosphere! So why is only the dust responsible for the blue colour of the sky!! The funny thing is, Tyndale R.The Reilly duo, however, thought that vapour, dust, everything was responsible for the change in the colour of the sky. But later the scientists found that if the vapour was also the cause of the scattering, then the colour of the sky would look different in one place due to the difference in humidity, fog, etc. At the same time, they came to the conclusion that the oxygen and nitrogen molecules at different levels of the sky were enough to cause the sky to turn blue!

At the same time, when Einstein published his theory of scattering of light from molecules in 1911, this idea became even more entrenched, Let's get back to the main point, the shorter the wavelength of blue light, the more it is scattered, and so it is understood that the sky looks blue. Then the wavelength of the purple light ray is less than the blue light !!

Then why the sky does not look purple!! First, the scattering of light rays in the atmosphere is not constant for all wavelengths. At the same time, the atmosphere is so high above the earth's surface that much of that purple light is absorbed by the atmosphere as the scattered light reaches it. Second, the human retina contains three types of cone. They are most sensitive to red, blue and green light than other colours. We see different colours in the combination of the different proportions of the sensitivity of these cones. Our eye cells are not 'ultraviolet' light-sensitive. That is why the sky does not seem dark purple to people.