Blue marble Earth. Image credit: NASA
The first step to understand why is the sky blue, is to look at the composition of the Earth’s atmosphere. The atmosphere is mostly gases and a few other molecules and it completely surrounds the Earth. The most common gasses are nitrogen(78%) and oxygen(21%). The remaining 1% is made up of trace gasses, like argon, and water vapor. Let’s not forget the many small solid particles, like dust, soot and ashes, pollen, and salt from the oceans. The composition of Earth’s atmosphere varies, depending on many things like your location, the weather, and many other things. There may be more water in the air after a rainstorm, or near the ocean. Volcanoes can put large amounts of dust particles high into the atmosphere. Pollution can add different gases or dust and soot. The atmosphere is thickest near the Earth. It gradually thins out as you go higher and higher up. There is no sharp break between the atmosphere and space.
The next step in figuring out why is the sky blue is to look at the habits of light waves and the color of light. Light is a kind of energy that travels(radiates) in waves. Light is a wave of vibrating electric and magnetic fields. It is one small part of a larger range of vibrating electromagnetic fields. This range is called the electromagnetic spectrum. Electromagnetic waves travel through space at 299,792 km/sec which is called the speed of light. The energy of the radiation depends on its wavelength and frequency. Wavelength is the distance between the tops of the waves. Frequency is the number of waves that pass by each second. The longer the wavelength of the light, the lower the frequency, and the less energy it contains. Visible light is the part of the electromagnetic spectrum that our eyes can see. Light from a light source may look white, but it is actually a combination of many colors. Light can be split into its different colors with a prism. Nature is splitting sunlight into its different colors when you see a rainbow in the sky. The colors of the spectrum(rainbow)blend continuously into one another. The colors have different wavelengths, frequencies, and energies. Violet has the shortest wavelength in the visible spectrum. That means it has the highest frequency and energy. Red has the longest wavelength, and lowest frequency and energy.
Thirdly, in the quest to answer why is the sky blue, let’s talk about the action s of light in the air. As light moves through the atmosphere, it goes in a straight line until it bumps into a bit of dust or a gas molecule. What happens next depends on the wave length of the light and the size of the thing that it hits. Dust particles and water droplets are much larger than the wavelength of visible light. When light hits these large particles, it gets reflected, or bounced off, in different directions. The different colors of light are all reflected by the particle in the same way. The reflected light appears white because it still contains all of the same colors. On the other hand, gas molecules are smaller than the wavelength of visible light. If light bumps into them, it acts differently. When light hits a gas molecule, some of it may get absorbed. After awhile, the molecule radiates the light in a different direction. The color that is radiated is the same color that was absorbed. The different colors of light are affected differently. All of the colors can be absorbed. But the higher frequencies (blues) are absorbed more often than the lower frequencies (reds). This process is called Rayleigh scattering.
Now, I can give you an answer as to why is the sky blue. The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Very little of the red, orange and yellow light is affected by the air; however, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in every direction. So, no matter where you look, you see the scattered blue light. The next time someone wonders: ”Why is the sky blue?”, you have the answer for them.
Here on Universe Today we have a great article about why is the sky blue. Astronomy Cast offers a couple of good episodes: one is about atmospheres in general and the other is about another atmospheric phenomenon: auroras. Here is a final link about the light from the Sun.
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