Why is the sky blue?

Light travels in waves with different wavelengths. These different wavelengths make the light different colors. Short wavelengths make red light, while long wavelengths make violet light in the order red orange yellow green blue indigo violet. School children are often taught to remember this using the name Roy. G. Biv. White light, the kind you’re most used to seeing, is actually made up of all of the different colors of light.

When light travels from one substance, such as air, into another substance, such as water, it changes speed. This causes the different waves of light to spread out from each other. You can see this if you have a prism.

Purple and blue light are spread the most. So when light enters our atmosphere and is scattered, the blue waves scatter far and cover the sky. This makes the sky look blue.

When the Sun is close to the horizon at dawn and dusk, the red light that isn’t scattered as much can be seen. This is why sunrises and sunsets are often red and orange.

Another article that explains this in more detail