why is the sky blue

A curious child often asked the first question is: why is the sky blue? However, no matter how common this problem is, there are many misunderstandings and wrong answers: because it reflects the ocean, because oxygen is blue gas, because the sun is blue, and the correct answer is often overlooked. The reason why the sky is blue is because three simple factors are combined: the sun is composed of many different wavelengths of light, the Earth's atmosphere is composed of different wavelengths of light molecules, their size is different , Our eyes are also very sensitive to put these three things together, the blue sky is inevitable to show.

The sun is made up of all the different colors of light ... Our sun's light sphere is very hot, close to 6000 K, it emits a broad spectrum of light from the highest energy of the UV to visible, from purple to red, and then goes deep To the infrared part of the spectrum. The highest energy light is also short wavelength (and high frequency) light, while the lower energy of light has longer wavelengths (and low frequencies) than high energy. When you see a prism that splits sunlight into its various components, the reason for the light splitting is due to the longer wavelengths of the red light than the blue light.

The reaction of light at different wavelengths in our daily lives is very important and useful in response to material reactions. The large hole on the microwave oven allows short wavelength visible light to enter and leave, but to keep it longer for microwave light to reflect it. The thin coating on the sunglasses reflects ultraviolet, purple and blue, but allows long wavelengths of green, yellow, orange and red to pass through. These tiny invisible particles form our atmosphere - molecules like nitrogen, oxygen, water, carbon dioxide and argon atoms, all of which scatters light at all wavelengths, but more effectively scatters short wavelengths of light.

Because these molecules are much smaller than the wavelength of the light itself, the shorter the wavelength of light, the better the scattering. In fact, it follows a law called Rayleigh scattering, which tells us that the violet light in the short wavelength range of human vision is nine times more than the red light in the long wavelength range. (The scattering intensity of the wavelength is inversely proportional to the quadratic: I am αλ-4). When the sun shines on every corner of the Earth's atmosphere, the red light of the light will only have 11% of the scattering, so in your eyes like purple light.

When the sun is high in the sky this is why the whole sky is blue. It looks brighter far away from the sun because there are more atmospheres (and therefore more blue) in these directions. In any direction, you can see the scattered light from the sun, illuminating the entire atmosphere between your eyes and outer space. This has some interesting effects on the color of the sky, depending on the location of the sun and where you are looking.

If the sun below the horizon, the light must pass through a lot of the atmosphere. The blue light is scattered in all directions, while the red light is less likely to scatter, which means it will reach your eyes. If you fly by sunset or sunrise, you can see the spectacular view of this effect.

From the space description and the astronauts return the image, this is a better perspective.

When sunrise / sunset or lunar month / month sunset, the light from the sun (or the moon) must go through a lot of the atmosphere, the closer the horizon is, the more light passes through the atmosphere. When the blue light is scattered in all directions, the red light scattering efficiency is much lower. This means that the two lights from the sun or the moon's disk itself into a red color, but also from the nearby sun and the moon's rays - hit the atmosphere and scattered light to reach our eyes - before only one priority famous.

In the total solar eclipse when the shadow of the moon falls on you, blocking the sun directly to the large area around you, the horizon will become red, not somewhere else. In the eclipse of the path, the light is illuminated in all directions, which is why in most places the sky is still blue. But near the horizon, those scattered in all directions of the light is likely to reach your eyes before the scatter again. The red light is most likely to pass through the wavelength of light, eventually exceeding the more efficient scattering of blue light.

So nevertheless, you may have another problem: if the light of the shorter wavelengths is more effectively scattered, why the sky does not show purple? In fact, there are more violet light from the atmosphere than the blue light, but there are other The color of the mix. Because your eyes have three types of cones (for detecting colors) and monochromatic rods, which are all four signals that need to be interpreted by your brain when assigning colors.

Each cone, plus the rod is very sensitive to different wavelengths of light, but they are to some extent by the sky to stimulate. Our eyes are more intense against blue, cyan and green wavelengths than violet. Even if there is more purple, is not enough to overcome our brain to pass a strong blue signal.

This is a combination of three factors:
1, the sun is composed of many different wavelengths of light
2, the particles in the atmosphere is very small, scattered short wavelength wavelength of light than the wavelength of light more effective
3, our eyes have a variety of colors are responsive
This makes the sky blue for mankind. If we can see the UV more effectively, the sky will appear more purple and ultraviolet; if we have only two types of cones (like dogs), we can see the blue sky during the day, not red, orange and sunset The yellow. But do not be fooled: when you look at the earth from the sky when it is blue, but the atmosphere has nothing to do with it!
Author: Ethan Siegel (astrophysicist)