Ask A Scientist
Why Does Magnet Makes Color on TV?
Asked by: Hector Osorio
School: Woodrow Wilson School, Binghamton
Teacher: Angela Stephens
Hobbies/Interests: looking at and learning about space
Career Interest: astronomer
Answer from Jian Wang
Academic areas: Condensed matter and magneto-electronics
PHD School :University of Science and Technology of China Family: Wife, one daughter (11) and one son (8)
Intersts/Hobbies : Outdoors, board games
Our world is full of colors and images made by various things. Dancing fire adds joyful colors to a cold evening outing; a firework adds wonderful colors and patterns to a festival night sky; a paintbrush adds amazing colors to an oil painting; and a soap bubble display rainbow bands of seven colors. But, a magnet along does not make any color to most of things around it, and yet when we placed it next to a television various colors are added to the world that is shown by the TV screen. What is going on here? This question in fact has two parts: First, how does TV work to make colored pictures to show the colorful TV world as it appears; second what does a magnet do to this colorful imaginary world? A TV in our living room is in fact, for most part, a giant vacuum tube with a large flat panel serving as the blackboard within which the imaginary world emerges when various colors of lights are emitted. The inside of the flat panel is painted with a series of very narrow vertical strips, consisting of red, green and blue phosphors. When a strip is struck with an electron beam it emits its corresponding color at the spot where it is struck. In the back of the TV tube there are three electron guns that shoots out three narrow beams of electrons (also called cathode rays). Each gun only excites one phosphor with one color by having its special striking angle selected by a mask. The electron beams from the other two guns can never reach strips of this one color blocked by the mask. The three phosphors, lighted in different combinations of intensity, can create any visible color when viewed from even a slight distance. To form a color picture the three electron guns are swept across the screen very rapidly to excite different strips with different intensity to give off different combination of lights of different colors on the screen. When these colored spots are added together emerges is a colored picture on the TV screen. The electronics in the TV unit controls the precise intensity at each spot as the electron beams scan through the screen to form desired color picture. Now, what would happen if we bring a magnet next to the TV screen? The short answer is that the magnet will bend the electron beams at different spot. A magnet carries an invisible magnetic field around it. This is how it can attract other magnets and metal objects and why it sticks to a metal cabin and refrigerator. The magnetic field also exerts a force to a traveling electron beam. The exerted magnetic force is such that it is always perpendicular to the traveling direction of the beam, thus a magnet always pulls an electron beam away from its original traveling direction. In fact, this is how the electron beams in the TV tube are made to scan across the screen: There is a series of electromagnets, called a yoke, which draw the beams horizontally across the screen a line at a time. Now, when you put a magnet next a the TV screen, its magnetic fields bend the electron beams to make them travel in a different direction as originally intended, thus messing up the colors of the pictures. So, it’s not so much that a magnet creates colors but that it distorts the colors of the intended picture on the TV. Now try the following, turn on your VCR but without putting in a tape, your TV will have a blue background only (mine does, anyway). In this situation, only one electron beam is turned on – the one to strike blue phosphor strips. If you place a small magnet on the screen you will see that one side of the magnet show reddish color while the other shows greenish color. This shows that the two ends of the magnet has magnetic fields with opposite directions (opposite poles) and thus bending the same electron beam in opposite directions. In one end it allows the electron beam to strike the red phosphor strips by bending it one way, while the field at the other end bends the electron beam oppositely to strike the green phosphor in the pixels. When you bring in a very strong magnet, you will see wonderful patterns of different colors radiating out around it. This is certainly what you were looking for as you turn on the TV, but the magnet does create a wonderful virtual world of its own. You also may notice that if your magnet never produces any color in a dark region on the TV screen, say, when it is showing a dark night. Why is this the case, then? I will let you figure that one out.