The sky looks blue because air molecules scatter short blue wavelengths more than long red ones.

The longer version

Air molecules are far smaller than a wavelength of light, and such tiny particles scatter short waves much more efficiently than long ones. Blue and violet light bouncing out of the sunbeam in all directions fills the sky with scattered light, and the eye reads that diffuse glow as blue.

Violet scatters even more strongly than blue, but sunlight carries less of it and the eye is less sensitive to it, so blue wins. At high altitude, with less air above, the sky deepens toward a darker shade. Astronauts report a black sky, since there is no atmosphere to scatter anything.

Where this entry sits

These facts explore how light behaves and how human eyes turn it into the colors we see. Along the way you meet rainbows, prisms, mirrors, shadows, lasers, and optical tricks that fool our vision every day. The full list sits on the Color and Light page, where every entry is listed in order.