Why is the sky blue?

In order to understand why the sky appears blue during the day, one must understand some general principles about light. The white light released by the sun actually contains a mix of all of the colors of light the human eye can see. This is known as the visible spectrum. This can be observed by shining light through water, which causes the light to scatter. The observer is able to see the light in its various visible colors. This is known as the Tyndall effect, first discovered by John Tyndall in 1859. 

The Tyndall effect is also why the sky appears blue during the day. During the day, the sun is at an angle relative to the viewer on the ground here on Earth. The blue light is scattered more than all the other colors. Because of this scattering by molecules in the atmosphere, when one looks up at the sky it appears blue. Interestingly, when the sun is further away relative to the viewer, such as at sunrise or sunset, the blue light is scattered so much that the observer now sees the scattering of red and orange light instead of blue.

Hope this helps!