One of the most commonly seen sights is the blue sky, yet what’s not widely known is what makes it blue .

The sun emits light that travels through space toward Earth . Because space is a vacuum (i .e ., it has no atmos-phere), the light remains largely undisturbed until it nears the Earth, whose atmosphere is made up of a mixture of gas molecules (mainly oxygen and nitrogen) and other materials . The closer you get to the Earth, the thicker the atmosphere .

Light from the sun appears white but is in fact a com-bination of colors, and the range of these colors that are visible to the human eye—from red to violet, by way of or-ange, yellow, green, blue and indigo—is shown when light is passed through a prism . These different colors have different wavelengths and energies, with violet having the shortest wavelength and highest energy and red having the longest wavelength and lowest energy .

As light hits the Earth’s atmosphere, the different colors react in different ways . Some of them get absorbed by the gas molecules while others do not . Most of the longer-wave-length colors (such as red and orange) pass straight through the atmosphere and are unaffected, while many of the shorter-wavelength colors (such as violet and blue) get ab-sorbed by the gas molecules, because the wavelengths (i .e ., the distance between the peaks of each wave) of these colors are similar in size to the diameter of an atom of oxygen . The gas molecules then radiate these colors and scatter them across the sky, causing the sky to appear blue .

The reason why we perceive the sky as blue and not violet is because our eyes are more sensitive to blue .

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