Rayleigh Scattering
Rayleigh scattering is the way tiny bits of air bend and spread short waves of light more than long ones.
Drop a tiny pinch of flour into a clear glass of water and shine a light through it. Short blue waves bounce off the flour bits and spread in all ways. Long red waves pass straight through with less bouncing. This is why the sky looks blue — short waves scatter more than long ones.
Explaining rayleigh scattering by grade level
Sunlight looks white, but it holds all the colors. When light hits the tiny bits of air, blue light bounces around more than red light. That is why the sky looks blue during the day. At sunset, light goes through more air, so the sky turns red and orange.
Projects that explore rayleigh scattering
Sunlight looks white, but it contains every color mixed together, each traveling as a wave with a different length. When sunlight enters the atmosphere, gas molecules scatter blue light much more than red because blue has a shorter wavelength and bounces around more easily. As a result, scattered blue light reaches your eyes from every direction overhead. You can see this at home: add a little milk to a glass of water and shine a flashlight through it. The water looks bluish from the side, and reddish when you look straight at the light.
When light travels a longer path through a scattering material, short blue waves get bent away first and longer red waves pass through. You can observe this by shining a flashlight into a clear glue stick: the end near the light glows blue, while the far end glows yellow-orange. Taping more glue sticks together lengthens the path, and the colors shift further toward red, modeling how sunsets look red because sunlight crosses a thicker layer of atmosphere.