[SciShow intro plays]   Michael: There are plenty of unconventional boat and plane designs out there. But you have to admit that the Zumwalt-class destroyer is one weird-looking boat. And sure, the B-2 Spirit is sleek, but it’s pretty unusual for a plane to be shaped like a triangle with a bunch of pieces chopped out of one side. But there’s a good reason why the Zumwalt and B-2 look so strange: they’re designed for stealth.   There are two main ways to detect objects in the air and sea: radar and infrared. And stealth vehicles are designed to avoid both of those systems. The most important thing stealth vehicles have to do to stay hidden is literally slip under the radar.   It’s also the reason they have all those clean lines and sharp angles. The physics behind radar, or Radio Detecting and Ranging, isn’t too complicated: A transmitter sends out an electromagnetic signal in a specific direction. You might think it would be a radio wave, since the first “r” in “radar” stands for radio, but modern systems generally use microwave signals.   The wave then travels through space until it reaches an object, like a plane or boat. Then it bounces off that object, and some of the signal gets reflected back to the radar system. Electromagnetic signals travel at a constant speed, and if you multiply speed by time, you get distance.   So if you know how fast the signal is traveling, and you know how long it took to hit the object, bounce off, and then hit the detector, you can figure out the object’s location. With a more detailed analysis of the reflected signal, you can even figure out how fast the object is moving.

So, radar is helpful for keeping track of things like planes in the sky and boats in the ocean. But sometimes, militaries want to be able to spy or sneak up on their enemies. Which is why we have stealth planes and boats that can slip by radar detectors, either because they’re specially shaped or because they use radar-absorbent materials. See, an object’s shape has a lot to do with how a radar signal gets reflected.   A Boeing 747, for example, is a nice, fat, round plane. When a signal from a radar detector hits it, a lot of the signal gets reflected back. But planes and boats that are designed for stealth deflect the signal away from the radar system rather than back toward it.   It’s like how if you throw a ball at a flat, horizontal table, it’ll hit the top of the table and bounce away from you instead of back to your hand. Your hand is like the radar system and the table is like the flat surface of a B-2. Most of the signals never make it back.   There are a few small curves on the B-2, and any radar signals that hit those surfaces will reflect back. But it’ll be a weak signal, and it’ll be coming from a few small, scattered parts of the plane. So instead of looking like a massive bomber plane, it’ll look like a flock of birds or something.   There’s also another way to make stealth planes and boats undetectable to radar: coating them in materials that absorb radar. One radar-absorbing material that’s used a lot is Iron Ball Paint. You can probably guess what this paint is made of: microscopic spheres with an iron-based coating.   When a radar signal hits the iron ball paint, the paint ends up with a changing magnetic field. That makes the molecules in the paint oscillate back and forth, which converts the energy from the radar signal into heat. That heat then gets transferred to the rest of the plane, where it dissipates.   As more radar signal gets absorbed by the paint, less gets reflected back to the radar system, and the harder it is to detect whatever’s coated in the paint. But extra heat can also be a problem for stealth vehicles, because another way to detect them is through their heat signature. Almost everything emits some amount of infrared radiation.   And hotter objects, like a plane putting out lots of exhaust, will emit more. Infrared detectors keep track of incoming infrared radiation and can find planes based on the heat they give off. Which is why stealth planes try to keep their parts and exhaust as cool as possible.   There are a few different ways for planes to cool down, but a lot of the time, they’ll just mix the hot exhaust with cool air before it’s released. With a less intense heat signature, the plane becomes more difficult to detect. So a huge blocky ship or a big dark boomerang-shaped plane might be easy to spot with your eyes. But to a radar or infrared detector, they’re practically invisible.   Thanks for watching this episode of SciShow, brought to you by our patrons on Patreon. If you want to help support this show, you can go to Patreon.com/SciShow. And don’t forget to go to YouTube.com/SciShow and subscribe!