I was recently in Hitchcock Hall at The Ohio State University with my daughter. Displayed in the lobby is a jet engine used in the Boeing 737.

hitchcock hall display

To the left and the right of the jet engine were two mounted flatscreen monitors giving information about the engine and its operation. We started talking about these two inventions, which both in their own ways changed the world. My daughter made the point that in fact the flatscreen and everything it represents might be a more important invention than the jet engine, because it is the flatscreen and the explosion of information technology that has truly opened the world. It’s a good point. They’re both such amazing inventions, though, that I wanted to write a little about each of them.

How does a jet engine work?

What I’ll write here completely ignores the most important parts of a jet engine – the control systems, the sensors, the subtle ways in which we humans get the engine to do our bidding. Just as Wilbur and Orville Wright’s great contribution was not the physics of flight but the control of the airplane, the thing that makes a jet engine work is the way we monitor and control it. But I don’t know nearly enough to write anything about that.

What I do know is the basic physics of the engine. Much like your car’s engine, the jet engine burns fuel. As the fuel burns, heat is released, and gases expand. Unlike your car’s engine, though, the expanding gases in a jet engine don’t push on a piston. Instead, they push on the blades of a turbine, causing the turbine to spin around 1o,000 times a minute. The turbine is connected via a shaft to the huge fan at the front of the engine. This spinning fan pulls in air. Some of this air is compressed and pushed into the combustion chamber to support the burning fuel. Most of it, though, bypasses the combustion chamber and comes flying out the back of the engine, producing most of the engine’s thrust.

Here’s an animation from NASA showing the key pieces:

engineanimated

This air, mixed with burned fuel and air from the combustion chamber, flies out the back of the engine. That’s the action. The reaction is that the engine (and the aircraft it’s attached to) moves forward, into even more air. And the world flies.

OK, what about the flatscreen?

The same caveats apply. Ask me to build, or even fix, a flatscreen, and I’m lost. The true genius of modern flatscreen monitors lies not in the basic physics, but in the control, the logic, the functionality of the screen. And of these details I’m painfully unaware. Again, what I know is the physics.

Not long ago, televisions were electron guns. Electrons produced at the back of the television flew through a tube (the cathode ray tube) and struck the phosphors at the front of the screen, producing light. The technology worked, but the tubes were heavy, expensive, and used a lot of energy.

Today’s flatscreens depend on two technologies that sound similar but are in fact quite different. The light energy emitted from a flatscreen into your eye originates with light-emitting diodes, LEDs. The particular colors produced by the screen come from liquid crystal displays, or LCDs. Here’s a little about each.

LEDs are light bulbs that don’t use a filament. Instead, they use a sort of cliff, over which electrons fall. When electrons move from one material to another inside the LED, they fall into a lower energy state. You can imagine the electrons jumping off a cliff, shouting “cowabunga” or something as they fall, except the “shout” comes out as a piece of light, a photon. The difference in energy between the beginning material and the ending material determines the type of photon released. Phosphors in the LED turn those photons, usually of a very specific type, into a wide spectrum that our eyes see as white light.

band gap cliff

LCDs are completely different. These in a sense reverse the action of the phosphor inside the LED. By absorbing photons of many colors and emitting photons of just one color, each liquid crystal can make a single dot of a single color on the screen.

This is what an LCD flatscreen looks like up close.

This is what an LCD flatscreen looks like up close.

The amazing thing, though, is that each color cell in the screen can be (and is) turned on and off very quickly with just a small electric signal, allowing each tiny piece of the screen to produce a particular color at a particular time. When added together, all these on and off signals add up to the picture. And the world sees.

Two world-changing technologies on display side by side. Ain’t science grand?

 

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