The last chapter ended with a warning: standing tall above the noise floor is not enough, because the space between two antennas is almost never empty. It has drywall and brick, windows and rain, hillsides and hot soup, and you. Yet a radio wave clearly gets through some of it, or your phone would only work outdoors with the tower in plain sight.
So what actually stops a wave? The everyday evidence is confusing. Light sails through glass but not through a wall; Wi-Fi sails through that same wall but dies in the bathroom; AM radio shrugs at an entire hill. The answer is one of the tidiest in the book, and it starts with the three things that can happen when a wave meets stuff.
Three fates#
When a wave arrives at a slab of anything, every bit of its energy meets one of three fates. It can pass through, the way light passes glass and radio passes your body, barely noticing. It can be soaked up, its energy staying inside the stuff and turning into warmth; that is not a malfunction, it is exactly how a microwave oven cooks: water and the watery parts of food happen to drink in waves around a few gigahertz, so the oven fills its little metal room with them and the soup does the rest. Or it can bounce back, the way light bounces off a mirror. Metal is radio's mirror: its loose electrons swallow the incoming wiggle and instantly re-fling it, so almost nothing gets inside.
Real stuff deals every wave a mix of all three fates: a bit bounced, a bit soaked up, the rest let through. The mix is what we mean by "blocking".
Pass through. Glass barely notices light; walls barely notice AM radio.
Be soaked up. The wave's energy stays inside and becomes warmth. Water loves to do this.
Bounce back. Metal is a mirror for radio; almost nothing gets in at all.
The wall test#
Which fate wins depends on two things: what the stuff is, and how fast the wave wiggles. That second part is the surprise. The very same wall can be nearly invisible to one frequency and a dead end to another. As a rule of thumb for the airwaves, the faster the wiggle, the harder the world is to get through: masonry charges a steeper toll as frequency climbs, and water goes from bystander to sponge somewhere in the gigahertz range.
Try it. Pick a material, slide the frequency, and watch where the wave's strength goes. Send an AM wave at the brick and it barely notices; send 5G at the same brick and hardly anything comes out the far side. Then try water at Wi-Fi's 2.4 GHz and you'll see why your soup steams, and why the signal drops in the bathroom, a small room lined with tile, pipes, and plenty of water.
The box that traps a wave#
That metal mirror explains a small everyday miracle: the window in your microwave oven. The oven is a sealed metal box precisely so its waves bounce around inside and never leave. But then how can you watch your popcorn through the door? Look closely at the glass and you'll see a fine metal mesh punched with little holes. Here is the clever part: to a wave, a hole smaller than its own wavelength barely counts as a hole at all. The oven's waves are about twelve centimeters long, the width of your hand, and to something that size a few-millimeter hole is a solid wall; the wave cannot squeeze through and bounces back to work. Light, whose waves are thousands of times shorter than the holes, sails through and lands in your eye. Same door, two frequencies, two verdicts.
A box of metal that waves cannot enter or leave has a name: a Faraday cage , after Michael Faraday, who built a room of foil and wire in 1836 and sat calmly inside it while electricity crackled around the outside. You step into an accidental one every time an elevator's doors close and your call dies, and a building wrapped in foil-backed insulation or metal roofing can be a big, unintentional cage too. Nothing mysterious is happening to your phone; it is simply shouting inside a mirror box.
Long waves bend, short waves shadow#
One puzzle is left: the hill. No tunnel runs through it, yet AM radio arrives on the far side. The secret is not toughness but size. Remember from chapter 3 that low frequencies mean enormous waves; an AM station's wave is longer than a football field. Waves bend around obstacles smaller than or similar to themselves, the way ocean swells wrap around a boulder and close ranks again behind it (engineers call the bending diffraction). To a three-hundred-yard swell, a hill is a pebble in the surf. The longest radio waves even hug the curve of the ground itself, a low crawl engineers call the groundwave , following the planet over the horizon.
Some AM waves pull an even grander stunt after dark. High above you floats the ionosphere , a layer of air electrified by the sun, and at night it turns into a gentle mirror for AM frequencies while the lower, absorbing air below it calms down. Waves skip off the sky and land hundreds of miles away, which is why your car radio can suddenly pick up a station from two states over at midnight that vanishes again at breakfast.
Short waves get none of these favors. A Wi-Fi wave is a few inches long, so an ordinary wall is, to it, a vast cliff face: no lapping around, just a crisp shadow behind, like a flashlight beam blocked by a book. This is also the trade hiding in your router's two networks: 2.4 GHz waves are longer, bend and seep a little better, and reach the garden, while 5 GHz carries more data but acts even more like light, happiest in the same room.
An AM station's waves are hundreds of yards long: they roll over the hill like ocean swells around a boulder, and the house behind it still hears the show.
Wi-Fi waves are just a few inches long: they behave more like light from a flashlight, and a solid wall leaves a crisp dead zone behind it.
The gauntlet, complete#
So the journey between antennas is a gauntlet with rules. Distance thins a signal by the square. The noise floor hisses beneath it. And everything in between deals it the three fates: a bounce here, a soaking there, the survivors passing on through. Which fate wins is set by frequency, and the family resemblance to light grows as the wiggle speeds up.
But one obstacle story is left, and it is the strangest in the book. A few paragraphs ago a mirror slipped past almost unnoticed: that electrified layer of sky that lets AM stations skip across whole states at midnight. It is a wall hanging over the entire planet, built fresh every morning by the sun itself, and the moment the sun goes down it turns the sky into a hall of mirrors. That story gets the next chapter to itself.