Ever taken a look at the sky during the day? Notice how it's blue? Good, so you're not crazy, but you are seeing things. While we have always perceived the daytime sky as "blue", according to scientists, the sky is actually in fact "purple." It's the ingenuity of the human eye and brain that makes us think and see otherwise.
As you already know, we receive our light from the Sun. While light appears "white", it is actually made up of a spectrum of colors. When that light is bent, those colors can be seen. This is why our sky appears blue and not red, yellow, or green. The light from the Sun is bent by molecules in our atmosphere, which causes the light to scatter. This is called Rayleigh scattering and it is more prominent with the blue/violet end of thespectrum. The fact that we even have a daytime and a colored sky is because Earth has an atmosphere. Mercury, the closest planet to the Sun, is completely dark because there is no atmosphere to bend and scatter light.v Red light contains longer wavelengths while blue and violet light are shorter. Shorter wavelengths are scattered more, so during the day, the blue wins. Sunsets and sunrises appear red because the Sun is magnified being so close to the horizon, which means the waves of red light win. This picture from Live Science simply explains it.
So since the shorter the wavelength means the more scattered the light, wouldn't purple win out over blue? Correct, and is does; the sky is purple, but the reason we don't see it that way is simply the structure of our eyes. According to Forbes and the Rochester Institute of Technology, our eyes contain rods and cones which allow us to process light and color. The cones in our eyes are most sensitive to short, medium, and long wavelengths, which is the same as saying blue, green, and red.
Our blue-sensitive cones would be stimulated the most by blue light, but the green and red cones experience a little stimulation as well. If the sky truly was blue, then to our eyes it would appear as greenish-blue. While violet is the most scattered color in the sky, our eyes are not as sensitive to violet as they are blue. Yet our red cones are actually more sensitive to violent than blue, so if red cones dominated our eyes, violet with a tinge of red is what we'd. When all 3 types of colors are combined: the reddish tinge of violent, the bluish green tinge, and blue and violent, it all equals a pale blue sky that we have come to know and love. This all also means that animals perceive colors differently as well since the types of rods and cones vary among animals.
Don't freak out now and think that you have been lied to all your life. The sky may in reality be purple, but it's likely not as deep of a purple that you would imagine. Most ultraviolet light is blocked by the atmosphere, and the Sun lets off less violet light than the other colors. Still, it's a fascinating phenomenon, and it's also not unique Earth.
*picture from BBC, links from Forbes, Rochester Institute of Technology, Live Science, HyperPhysics
