What is a microwave?

0:00 Welcome to Science is Not Boring by Kidopoly.com. I'm Mira. And I'm Finn. A huge hello to Elliot and Jonah, who love listening to the podcast together. Jonah actually asked us how microwaves work. So this incredible episode is for you, Elliot and Jonah.

0:18 And a big hello to Nico, 6, Aurelia, 5, and 2-year-old Mila, listening in the car in Sydney, Australia.

0:27 If you haven't already, tap follow wherever you're listening so you never miss an episode.

0:33 Okay, Mira. Jonah wants to know about microwaves. What is so special about that box in our kitchen?

0:40 Oh Finn. Imagine a machine that shoots totally invisible waves. And those waves vibrate 2.45 billion times every single second. Wait, billion? With a B? Yes! These invisible waves pass straight through glass and ceramic plates like ghosts. But when they hit your cold food, they heat the outer layers using pure friction, spreading warmth inward.

1:09 No way! It sounds like a sci-fi laser beam. Not something I use to melt cheese on my nachos.

1:15 It totally does! Today we are cracking open the science of the microwave oven. We are going to find out why it makes metal spark, why your bowl stays cool while your soup gets boiling hot, and the weird accident that started it all. So, where did this machine even come from? Did a chef invent it? Nope.

1:35 It was discovered by accident by a self-taught engineer named Percy Spencer, and it happened way back in 1945. 1945? That's over 80 years ago. Exactly! Percy worked for a company called Raytheon in Massachusetts, but he wasn't studying food. He was building radar systems to help airplanes detect things far away.

2:00 Radar, like the spinning green screens that go beep, beep, beep. Yes! To make those radar systems work, Percy used a powerful device called a magnetron.

2:12 Ha, magnetron? That sounds like a giant robot villain from a movie.

2:18 It really does. But a magnetron is actually a heavy metal vacuum tube that takes normal electricity and converts it into electromagnetic waves.

2:28 What kind of waves are we talking about here? Like ocean waves?

2:32 No, these are part of the electromagnetic spectrum. It's the same family as radio waves, X-rays, and the visible light we can see with our eyes.

2:42 Wait, so the microwave is just blasting out a special kind of invisible light?

2:47 Exactly! One day, Percy Spencer was standing right in front of an active radar set. The magnetron was humming, pumping out invisible microwaves.

2:57 Was that safe? Did he get superpowers?

3:01 He didn't get superpowers, but he did get a messy pocket. You see, Percy had a peanut cluster chocolate bar in his pocket, and while he stood near the magnetron, he noticed something weird.

3:13 Don't tell me! The chocolate melted!

3:16 It melted into a warm, gooey mess. He hadn't felt any heat on his skin, but the candy bar was totally liquefied. And that gooey pocket changed history forever.

3:27 Okay, so if my chocolate bar mysteriously melted in my pocket, I'd probably just be sad. What did Percy Spencer do?

3:35 Because he was a brilliant scientist, he immediately ran a test. He sent a friend out to grab some unpopped popcorn kernels.

3:44 Oh, I see where this is going.

3:47 Percy placed the popcorn near the magnetron tube. Suddenly, the invisible waves hit the kernels and pop, pop, pop! Popcorn flew all over the laboratory floor.

3:59unknown Haha!

4:00 That is the best science experiment ever! Did they try anything else?

4:05 They did. The next morning, Percy brought in a raw egg. He cut a hole in a kettle, put the egg inside, and aimed the magnetron right at it. Uh-oh. Eggs in extreme heat usually end badly. You are so right. Another scientist leaned in to look closely just as the egg rapidly heated up. The pressure built and BAM!

4:30 The egg exploded right in his face. Oh no! A hot gooey explosion! He was okay, just very messy. But Percy realised something huge. The magnetron wasn't heating the air in the room. The invisible waves were penetrating the food's outer layers, letting the heat travel directly inward. But how? What are these waves actually doing to the food?

4:57 It's all about water. Almost every food on Earth, whether it's an egg, a piece of chicken, or a potato, has water molecules inside it. And water molecules are polar. Polar? Like polar bears? Sort of like a magnet. A polar molecule has a positive end and a negative end. When a microwave hits these water molecules, it acts like a wildly spinning electric field.

5:23 Inside the oven, the magnetron is blasting waves that vibrate at 2.45 GHz. That means the waves flip back and forth 2.45 billion times per second. 2.45 billion times? Every single second? Exactly! So all those tiny water, sugar, and fat molecules inside your food try to keep up. They flip back and forth, rubbing against each other billions of times a second.

5:54 Oh, I know what happens when things rub together super fast. Friction! If I rub my hands together quickly, they get hot.

6:02 You nailed it, Finn. That molecular friction generates intense heat deep inside the food. It's totally different from a regular oven, which just blows hot air around the outside of your food.

6:14 Wait, if the waves heat up water, why doesn't my ceramic plate melt? Or my glass bowl?

6:20 Because glass, plastic and ceramic don't have water in them. To a microwave, a glass plate is practically invisible. The waves pass right through it without moving the molecules.

6:32 That is mind-blowing. But wait, what about metal? My dad always says never ever put aluminum foil or a metal fork in the microwave.

6:42 Your dad is very smart. Metal is full of free-floating electrons. When microwaves hit metal, they don't pass through it and they aren't absorbed. Instead, metal reflects the waves like a mirror. And reflecting waves is bad? Very bad. The waves shove the electrons around so violently that they bunch up on the sharp edges of a fork or crinkled foil.

7:07 The electrical voltage gets so high, it actually rips electrons right out of the surrounding air. Whoa! What does that look like? It creates a burst of incredibly hot plasma. It looks like miniature lightning bolts shooting off the metal, throwing sparks everywhere. It can totally fry your microwave and even start a fire.

7:31 Okay, no metal. Check. But I have another mystery for you. Why is it that when I microwave a hot pocket, one side is boiling lava and the middle is frozen solid?

7:44 Ah, the dreaded hot pocket ice core. That happens because of something called standing waves. Standing waves? What does that mean? Think of a jump rope tied to a tree. If you shake the end, waves travel down the rope. But if the waves bounce back from the metal walls of the microwave, they crash into each other.

8:05 So they jam up? Exactly. In some spots, the waves combine and become super powerful, creating blistering hot spots. But in other spots, they cancel each other out completely, creating cold, dead zones.

8:21 Oh, is that why almost all microwaves have a glass plate that spins in circles?

8:27 Yes, the turntable drags your food through the hotspots and the cold spots, evening out the heat. Before spinning plates were invented, people had to stop the microwave and turn their food by hand every minute.

8:42 That sounds exhausting. Microwaves are so small and convenient now.

8:46 They weren't always. The very first commercial microwave hit the market in 1947. It was called the radar range, and you won't believe how big it was.

8:57 Was it the size of a dishwasher?

9:00 Bigger. It was nearly six feet tall, weighed 750 pounds, and cost $5,000. That's more than $60,000 in today's money.

9:12 £750? That's as heavy as a full grown grizzly bear. Imagine having a grizzly bear in your kitchen just to pop some popcorn. Alright, Finn, time for the quiz. Are you ready? Okay, I'm ready. Hit me with those microwave facts.

9:27 Okay, first question. How many times per second do the waves in a microwave vibrate? Next one, this is tricky. What is the name of the heavy metal vacuum tube that creates the invisible waves? Question three. What was the first food that Percy Spencer noticed melting in his pocket? Alright, here's number four.

9:59 Why does metal cause a sparking plasma storm inside a microwave? Last question. Why do modern microwaves have spinning glass plates? Great job! Let's go through the answers. The answer to question one is they vibrate 2.45 billion times per second. For question two, the device is called a magnetron. Number three, the answer is it was a peanut cluster chocolate bar.

10:34 Question four. Metal reflects the waves, causing electrons to bunch up and rip through the air as sparks. And the final answer is to move the food through the hot spots and cold spots created by standing waves.

10:50 Wow, I am never going to look at my microwave the same way again. Just knowing there is a giant robot-sounding magnetron in there, making water molecules flip billions of times a second, is incredible.

11:03 Right? And to think it all started with a melted chocolate bar in a radar lab.

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11:43 We love hearing from you. See you next time on Science is Not Boring. Bye everyone.