The sky is dark without sunlight. The Sun is yellow or sometimes orange. So, why is the sky blue?
Sunlight is made up of all the colors of the rainbow: red, orange, yellow, green, blue, and violet. Well, may be we can find the answer from this clue.
The sky appears blue to us on a clear day, because the atoms of nitrogen and oxygen in the atmosphere separate the suns white light into its many colors, and scatter them throughout the atmosphere.
The wavelength of the blue light scatters better than the rest, predominates over the other colors in the light spectrum, and makes the sky appear blue to us.
Why Not Another Color?
The human eye has three types of light receptors, known as cones, located in the retina. The cones are either considered to be red, or blue, or green, based upon their strong response to light at these wavelengths. As light stimulates these receptors, our vision translates the signals into the colors we see.
When gazing at the sky, the red cones respond to the small amounts of red light scattered, and even less strongly to the orange and yellow wavelengths. Although green cones respond to yellow, their response to scattered green and green-blue wavelengths is stronger. Finally, colors near the strongly scattered blue wavelengths stimulate the blue receptors.
In short, the skylight stimulates the red and green cones almost equally, while stimulating the blue cones more strongly. For these reasons, our vision naturally adjusts as clearly as possible to separate colors.
