Talk about one of the most basic questions in life. You hear it when you are young from others (that is, if you haven't asked it yourself). When you get older, you hear others ask it. There are several answers that are usually said. One of the common answers is that it's a reflection of the water in the sky. Another is that it's just the color of the atmosphere. Another interesting answer is the same for why anything is any color: it's the color it absorbs and reflects all the rest (or vice versa). But really, the actual answer to this question is a surprisingly difficult.
First to understand this, you have to understand how color works. Color is simply wavelengths of light that leave an object and are interpreted by a sensory source, such as your eye. The wavelengths themselves can be reflected or scattered by an external source, or they may come from an object (like the sun or a light bulb). What colors are contained in a light source will determine what color is seen. For example, if you have red paint, and you shine blue light on it, the red paint will appear black. When you hold a prism up to the light, you will see the visible spectrum of light (red through purple). This is because the light entering the prism is separated into the different wavelengths. What makes these wavelengths different is the peak and valley of the wavelength itself. The purple side of the spectrum has short wavelengths, and the more you would move toward the red side would show you that longer the wavelengths get. Wavelengths that are shorter have more engery than longer ones, so it's easier to scatter the purple side than the red side.
Another thing you have to understand is about scattering light. Particles in the atmosphere, and even molecules and atoms can scatter light. However, as discussed above, color is a wavelength and can be scattered based on the frequency (the distance between peaks in the wave) of the wavelength. Light itself is a combination of all these wavelengths, so when light passes through atmospheric particles, the purple side of the spectrum scatters quicker and easier than the red side. If this didn't happen, then we wouldn't see a blue sky; we would see a black space. The reason these shorter wavelengths can scatter so much easier is because any wavelength that has a shorter frequency is thought to have more energy than one with a longer frequency, and this wave with more energy can bounce around particles easier than a wave with less energy.
So, taking that into account, if you look at the sun (which is not recommended), you will see white light. This is because you are looking directly at a light source, and you are receiving all the wavelengths of light. As you would look away, the sky would gradually turn into the blue we are used to seeing. The easiest wavelengths to scatter are the purple, indigo, blue side of the spectrum. So, if purple is the easiest to scatter, why isn't the sky purple?
That goes back to your eyes. Our eyes have millions of sensitive structures called cones, which detect the wavelengths of light. There are three types of cones, and each type is sensitive to specific wavelengths: one group sees the higher frequency, one group sees the middle frequency, and one group sees the low frequency. However, these cones overlap in their ability to see the frequencies. This means that although some cones are designed to see the higher frequencies, they can still see the lower frequencies, but they are much less sensitive to what they aren't designed for. Your eyes then detect different colors by blending them into one color instead of seeing all the parts of the whole. If you went to a concert, you could hear the different instruments, but your eyes would only see one color, and not the grouping of colors. Because of the way our eyes see colors, you see a combination of blue-purple mixed with white, which your eyes interpret it as light blue.
So, now you see why it's a hard question to answer. The whole answer is even longer, because the color of the sky also depends on the geometry of the particles through which light is being dispersed. If a child asks why the sky is blue, they probably won't understand the whole reason if it's explained to them: "it's a combination of the wavelength patterns being scattered through an obstacle which disperses different frequencies and your eye's ability to perceive these dispersal patterns." I don't know if that will make a lot of sense.
And one last thing: water is blue because it reflects the light of the sky. It can change based on the depth and the content. For example, deeper water will appear darker than shallow water, and a green color could be from algae or other organic matter, and brown could be from dirt, and red could be from rust.
Bet you didn't know that!
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