As you know, the sky is a panorama that opens from the surface of the planet, forming a certain appearance woven from light. Moreover, if you observe from different points, the sky may look different. So, from the porthole of an airplane that has risen above the clouds, a completely different view opens than from the ground at the same time. The clear sky is blue, but it is worth running into the clouds, and it becomes gray, lead or dirty white. The night sky is black. The reason for all such changes is light and its interaction with air and particles of various substances in it.
In order to say what color and why the sky is and why, you need to remember that color is just a section visible to the human eye in a fairly narrow range of the spectrum of electromagnetic waves. In this range, the boundary positions are occupied by red and violet colors (see Fig.), Gradually changing with the increase of the radiation wavelength into the so-called infrared and ultraviolet radiation, respectively. The light coming from the Sun to the Earth is seen as white, but as Newton proved, white light is a bundle of seven rays: red, orange, yellow, green, blue, blue and violet. The decomposition of light into the spectrum occurs when it collides with molecules of various substances, while some of the waves can be absorbed, and some can be scattered.
Many scientists have tried to understand why only the blue color in clear sky reaches us. The first hypotheses suggested the influence of ozone and water molecules. It was believed that they absorb red light and transmit blue, which gives the corresponding color to the sky. Subsequent calculations, however, showed that the amount of ozone, ice crystals, and water vapor molecules in the atmosphere is not enough to make the sky blue.
At the next stage of research, John Tyndall suggested that dust plays the role of the desired particles. Blue light has the highest resistance to scattering, and therefore is able to pass through all layers of dust and other suspended particles. Tyndall conducted an experiment confirming his assumption. He created a smog model in the laboratory and lit it with a bright white light. Smog has acquired a blue tint. The scientist made an unambiguous conclusion from his research: the color of the sky is determined by dust particles, that is, if the Earth’s air were clean, then not blue, but white heaven would shine above the heads of people.
The final point on the question of the color of the sky, put the English scientist, Lord D. Rayleigh. He proved that it wasn’t dust or he was able to color the space above his head in our usual shade. The point is in the air itself. Gas molecules absorb most of the light, and in the first place, the longest waves equivalent to red. Blue scatters.
Attentive people will notice that, following this logic, the sky above the head should be purple, since this color has the shortest wavelength in the visible range. The explanation is simple: at the same brightness, the human eye perceives the blue color better, rather than purple. In addition, green light, which also has some ability to scatter in the atmosphere, mixed with perfectly scattered violet, leads to the same blue. In fact, the blue we see is the result of mixing blue with violet and green.
Is it possible, using the same logic, to understand the bright red colors of sunset? Of course, one can even say that the sun on the horizon will be redder, the thicker the layer of the atmosphere lies between it and our eye, or the more restless and, accordingly, the more dusty this layer of the atmosphere will be. So, the redder the sunset, the more inclement and windy the next day will be. The fact is that during sunset and dawn, the sun’s rays have to travel a much greater path through the atmosphere than at the height of the day. In this case, the light of the blue-green part of the spectrum is strongly scattered or absorbed by the atmosphere and the clouds located at the horizon become colored in shades of red.
When the clouds cover the sky, the picture changes completely. The sun’s rays are not able to overcome the dense layer, and most of them simply do not reach the earth. The rays that managed to pass through the clouds meet with water drops of rain and clouds, which again distort the light, again mixing it. As a result of all these transformations, white light reaches the earth, if the clouds are small in size, and gray, when the clouds are covered by impressive clouds, which again absorb part of the rays.
This is how the reason for things that are quite familiar to us is revealed. So now we can just enjoy the contemplation of not mysterious, but still bottomless blue sky and divine scarlet sunset!
TASK: experience “Sky day and evening”
1) Pour in a jar half a teaspoon of milk. Add water to make a whitish liquid.
2) In a dark room, direct a beam of a flashlight to the side of the bank. The liquid in the jar will turn bluish.
3) Move the flashlight so that it shines directly at you through the can. Now the liquid looks red.
What’s happening? Droplets of milk in water, like air molecules in the sky, scatter light of different colors in different directions. This explains why at one time of the day the sky seems blue to us, and at another – red (at sunset).