Way way back many centuries ago, around 400 BC, philosophers started arguing about light. Did it have a finite speed, or was it variable like most things? This was back when science was first being invented, so philosophers didn't have too many facts to work on. 

Before we keep going, let's think about it. How would you test the speed of light? Is it finite or infinite? Does it need a material to travel through, or can it travel in a vacuum (the airless one, not the Hoover in your closet)?

Plenty of people had opnions on the topic, but they couldn't really agree on anything for over a thousand years. Then Rømer came around and figured out the speed of light and its finite-ness—by accident. Sure, his estimate was off by about 26%, but he got pretty close for the seventeenth century.

The speed of light in a vacuum, typically marked with a c, is about 3 × 10⁸ m/s, which is faster than

• a speeding bullet.
• the earth's rotation around the sun.
• literally anything in the universe.

That last bullet point was hypothesized by Einstein with his E = mc² equation, but that's a story for another day. (Unless you feel like taking a break right now to read our guide on it.)

The best part? Light doesn't need anything when it propagates. Unlike some waves (we're looking at you, sound), light is perfectly happy to travel through a vacuum or somewhat translucent materials. Because light does that, sunlight can actually reach the earth and give us energy.

Even though it's the only thing we can see, light's just a tiny part of the electromagnetic spectrum. All of the electromagnetic spectrum acts pretty similar to it, though. The speed of light is actually just the speed of electromagnetic waves; what changes is how quickly the waves loop and how long those loops are.

Radar's going to capitalize on those relationships, Shmooper, since it's a type of wave in that very spectrum.

(Source)