Why is the sky blue?
Imagine sunlight is made of many colors all mixed together — like a box of crayons. When sunlight comes to Earth it meets tiny bits of air and tiny particles. Those tiny bits bounce some of the sunlight in different directions. That bouncing is called "scattering."
- Sunlight has all colors. White sunlight is really lots of colors mixed together (red, orange, yellow, green, blue, indigo, violet). You can see them in a rainbow.
- Colors are different kinds of light waves. Blue and violet are waves that are shorter and wiggle more. Red and orange are longer waves.
- Air scatters the shorter waves more. The tiny air molecules spread out (scatter) the blue and violet light more than the red light. Because blue light gets scattered in all directions, when you look up you see blue coming from everywhere.
- We see the sky as blue. Your eyes see blue better than violet, so the scattered light looks blue to you.
Easy at-home experiment (with an adult):
Materials: a clear glass or jar, water, one drop of milk, a flashlight, and a white piece of paper.
- Fill the glass about 3/4 with water.
- Add one drop of milk and stir gently. The water should look only a little bit cloudy — not very milky.
- Turn off the room lights. Shine the flashlight through the side of the glass and hold the white paper on the opposite side.
- Look from the side of the glass where the flashlight enters. The light you see scattered to the side will look bluish. Now look at the light coming out the far side onto the paper — that light will look more orange or red.
What happened: The tiny milk bits act like air molecules and scatter the blue light to the side, so the side view looks blue. The light that goes straight through loses some blue, so the transmitted light looks redder — this is like what happens at sunset.
Why are sunsets red?
At sunrise or sunset the sunlight travels through more air to reach you (it goes across the sky). This long path scatters away most of the blue light, leaving the reds and oranges to reach your eyes. That is why the Sun and the sky near it look red or orange then.
Fun fact: If the air has lots of dust or pollution, sunsets can be even redder because more blue light gets scattered out before it reaches you.
That is why the sky is blue — because the air scatters the shorter (blue) colors of sunlight more, and those scattered blue rays come to your eyes from every part of the sky.