why is the sky blue?
The Real Answer
The sky appears blue because of a phenomenon called Rayleigh scattering. Sunlight contains all colors of the visible spectrum, but shorter wavelengths (blues and violets) scatter more easily when they hit nitrogen and oxygen molecules in the atmosphere than longer wavelengths (reds and yellows). We see blue instead of violet because our eyes are more sensitive to blue light, and the sun emits more blue than violet. At sunset, the light travels through more atmosphere, scattering away the blue and leaving reds and oranges.
Light has been traveling for 93 million miles to reach your face, and the first thing it does when it gets here is bounce off invisible gas molecules and change color. The sky isn't actually blue — it's just the sun trying to dodge eye contact with you.
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Why People Ask This
This is usually a kid's question that somehow nobody stops asking. You see the sky every single day, you assume you know why it's blue, and then one day you realize you have no idea and the explanation involves words like 'Rayleigh' and 'wavelength.' It's one of those questions that sounds simple until you actually try to answer it without sounding like a physics textbook.