Why is the sky blue?

Everyone knows the sky is blue, but not everyone knows why the sky is blue. Why is the sky often blue and not another color? In the sky, there is a very thick atmosphere covering the whole earth. That air is just a transparent color, so where does that "blue" color come from?

Explain why the sky is blue

Visible light (visible light) is part of the electromagnetic spectrum that the human eye can see. Light from the sun or electric light bulb is called white light.

Sunlight has 7 colors: red, orange, yellow, green, blue, indigo, purple. Each color corresponds to 1 wavelength, frequency and carries different energy. Purple light has the shortest wavelength in the visible spectrum. This means that the frequency and energy of violet light is highest in the visible spectrum. In contrast, red light has the longest wavelength, the lowest frequency and will carry the least amount of energy.

Light in the air

Light moves in space in a straight line if nothing makes it disturbed. As the light moves into the atmosphere , it continues in a straight line until it encounters tiny particles of dust or gas molecules . From this point on, what happens to light depends on its wavelength and the size of the objects it hits.

Airborne particles and dust are larger in size than the wavelength of visible light. When light hits larger particles, it will be reflected in different directions or absorbed by obstructions. Because the different colors in light are reflected from the particles in the same direction, the reflected light from the barrier particles is still white light and contains all the original colors.

In addition to dust and water, the atmosphere also contains gas molecules. These gas molecules are smaller in size than the wavelength of visible light. If the white light shines on the gas molecules, it is not as simple as when it comes to dust or water particles.

When light hits the gas molecule , its " part " can be absorbed by the gas molecule. After that, the gas molecules will radiate light in different directions from the original. The " partial " concept appears here because there will be a number of wavelengths in white light (corresponding to colors) that are easily absorbed, some other wavelengths that are more difficult to absorb. In other words, some short wavelengths (such as blue) will be absorbed more than long wavelengths (such as red).

British physicist John William Strutt, 3rd Baron Rayleigh (1842-1919) He proposed the equation to determine Rayleigh scattering coefficient to help explain the cause of sky blue

This process is called Rayleigh scattering . The phenomenon is named after its discoverer: Lord John Rayleigh, an English physicist. In 1871, Rayleigh gave an equation for calculating the scattering coefficient of an object inversely proportional to the wavelength of light (denoted by lamda). 4. In other words, the shorter the wavelength of light, the more dispersed. more radiation and vice versa.

It was possible to answer the original question: The sky's blue color was caused by Rayleigh scattering

Because the wavelength of light (100 ~ 1000 nm) is larger than the size of gas molecules (10 nm), we can apply Rayleigh scattering formula to the atmospheric light scattering phenomenon. Earth.

When light enters the Earth's atmosphere, most of the long wavelengths are not absorbed by gas molecules so it can pass through. A little red, orange, and yellow light can be affected by air. However, a large number of short wavelengths have been absorbed by gas molecules.The short wavelength light absorbed is then scattered out in many different directions.

At this time, blue light will scatter across the sky . In the daytime, no matter where you stand and look in the direction, some scattered blue light is directed towards your eyes. Therefore, when you look up above your head, the sky will always be green.

If you pay close attention, when you look closer to the horizon, the sky seems lighter. That's because, to get to your position, the blue light after being scattered must go through more layers of air. Part of it continued to be scattered in many other directions. Therefore, there is less blue light from near the horizon to your position than the amount of blue light from the top of your head.

Another noteworthy point is that if you follow up here, you will wonder: The wavelength of purple and indigo is even shorter than blue, so should the sky be purple? The answer is ready for you.

So why is the sky not purple? That is the shortest wavelength!

A major cause is the activity of the human eye in seeing colors. Human eyes are sensitive to light with a wavelength of 380 to 740 nm. On normal retina there are 10 million light sensing rod cells and 5 million cones detect color. Each cone cell contains pigments that respond to each type of wavelength. There are 3 main types of cone cells corresponding to short, medium and long wavelength types. We need to use all three types of cells to see the most accurate colors.

Each cone cell reacts with maximum wavelengths of 570 nm for long wavelengths, 543 for medium wavelengths, and 442 nm for short wavelengths. However, these three types of cone cells can react with a wide range of wavelengths and overlap. This means that there will be cases where two different spectra can cause the same reaction on cones.

Two different spectra, but the same reaction on the cones is called a heterogeneous isomer. Back to the sky problem, when the sky is a mixture of blue and purple. Cones in the human eye will react when they see this mixture into a mixture of blue and white . And finally, the signal to the nervous system is only blue. This is similar to mixing red and green to yellow.

However, some animals see the sky not as blue as humans. In addition to humans and certain types of primates, most other animals have two types of cones in the retina. Therefore, these animals, like birds , will see the sky as purple.

Why do we see the sun is yellow?

On Earth, we see the sun during the day in yellow. If you go out into space or up on the Moon, you will see the Sun is white. Why so? That's simply because: In the universe there is no atmosphere to scatter sunlight.

On Earth, several short wavelengths of sunlight (blue or violet) have been absorbed by gas molecules and removed from the beam of light directly from the sun to the human eye. Therefore, the remaining colors that appear together are yellow.

Finally: Why is the sunset red?

When the sun begins to set, the light needs to travel a long way through the air before reaching the location you see. At this point, more and more light will be reflected and scattered. The less light that comes directly from the sun to your location, the less the sun will glow. At the same time, the color of the sun begins to change, the yellow from daytime begins to gradually shift to orange and then to red.

The main reason is: Although the amount of blue light is still scattered like during the day, it is scattered many times because it has to penetrate thick air to reach the human eye. Besides, the long wavelengths (orange, yellow) in the light beam shone directly to your position less and less. Long wavelengths must pass longer distances than during the day to reach your position directly. Only the less scattered red light is transmitted directly to the eye.

Therefore, you will see the sky getting more and more red. After the Sun has disappeared from the horizon, we don't see the sun's light directly; but if there are clouds in the sky, they will reflect red light on the ground, creating a beautiful scene of the sunset.

Conclusion

Finally, we have found the answer to the original questions. Once again, the seemingly obvious phenomena hid in it many such problems. It is true that anything has its cause. Of course, people are still studying and trying day and night to try to explain more and more surrounding phenomena that previously had no answers. That is the wish of all of us and especially scientists. Everyone has many questions for themselves.