As the Sun’s light travels towards us, at first all it encounters is the emptiness of space. But here on Earth, swirling in the air around us are thousands of near-invisible particles. Dust and water droplets are the larger ones, but there are also the minuscule gas molecules—mostly oxygen and nitrogen—that make up the air itself. The wavelengths that make it to the Earth’s atmosphere collide with these particles, which sends light bouncing in many different directions. Think of it like this: If you’ve ever been in a crowded bumper-car pavilion at an amusement park, bumping into other cars and being bumped into yourself, you’ve played the part of a light wave.