How often do you watch TV? Television is one of those things that just about everyone uses. The average American teenager watches three hours of TV every day, and some kids can go four hours or more. Obviously, the moving pictures that appear on a TV screen are incredibly important to us. But how, exactly, do the pictures get there? How can you send pictures invisibly through the air, and then have them pop up on millions of screens all around the country? Let's find out!

In the Beginning
Television was invented in the 1920s, and it was a huge, monumental invention. It's hard for us to understand how amazing television was when it first appeared. We think of television as an everyday thing. But the idea of being able to take a moving picture in New York and transmit it instantly for hundreds or thousands of miles was (and is) amazing.

So how did they do it? What they had in the 1920s were radios and radio stations. Here's how radio worked. A DJ would play songs or talk into a microphone. The songs would get turned into radio waves. The radio waves would be "broadcast" from a tall radio antenna in the middle of the city. The radio waves would travel through the air at the speed of light. They would hit an antenna at each person's home. Then the radio in each home would turn the radio waves back into sound and play that sound -- be it music or a radio show – through a speaker.

That sounds great for music and radio talk shows, because we play music note by note and we talk word by word. Sounds are one-dimensional. But how do you transmit a two-dimensional moving picture with a one-dimensional radio signal? Philo Farnsworth is one of the inventors who solved this problem. The story goes that he found the answer while plowing a field at age 14. He looked at the furrows he was plowing, and he realized he could paint a picture on a screen line by line in just the same way. We still do it the same way today.

Here's what happens. A TV camera looks at the scene. It takes a picture of it. Then the camera scans across each line of the picture, and scans down line by line. The camera breaks each line into a series of dots. We also call them pixels. The camera figures out how much light and what color light it finds at each dot. Then the camera adds a special signal at the end of each line. The camera goes down to the next line, and goes through the whole image dot-by-dot and line-by-line. At the end of the image, the camera adds another special end-of-image signal. Then the camera starts on the next image. The camera does this at a rate of 30 images every second. Because the image is now a dot-by-dot stream, you can transmit it with radio waves, or send it through a one-dimensional wire.

At the receiving end, you have your television set. It might have an antenna, or it might hook into the cable TV system to get its signal. But either way, your TV is receiving the dot-by-dot signal from the camera. This signal tells the TV how bright, and what color, each dot on the TV screen should be. The TV screen lights up all the dots for one image, and then displays a new image 30 times every second. What you see is a moving picture on the screen.