The workings behind television screens

The Hindu

a day ago

After a few months of a hectic summer, the rains are here. The IIT Kanpur campus is green and nature's colours abound once more. With monsoon, however, comes alive the age-old tradition as well: Sunday evenings of guilt-free laziness, together with the music of the rain's patter, a Bollywood classic on the TV, and some hot, simmering tea.
Over the years, the world's technologies have changed shape and form, including the TV. Blinking tubelights have turned to LEDs and televisions have changed from being cubic boxes to flat screens. Why and how did this happen?
It has something to do with physics discoveries behind the scenes.
Electrons to light
When you switch on the TV, you really just switch on the electrical socket where the TV plugs in. We know sockets carry electric currents transported by electrons. But how do these electrons become light?
This isn't unusual if you think about it. We see it all the time in our houses. The protagonist of this puzzle is a class of materials called phosphors. The phosphors (which are different from the element phosphorus) are also called fluorescent compounds because they have something magical about them.
When an electron hits a phosphor, the material throws out light. This has to do with the way electrons are arranged inside these materials. When another electron falls on them, the electrons in the phosphor become excited to higher energies. When they relax back, they throw out some of that energy as light.
Phosphors are thus used to cover the insides of tubelights and fluorescent bulbs. It's the reason we call white bulbs 'CFLs', short for compact fluorescent lamps. Inside the bulb or tubelight, one just needs flying electrons or other charges to hit these materials. If you have ever seen an old broken tubelight, the powder inside the glass tube is nothing but phosphor.
Moving pictures
In a tubelight, since we just need the light, we can uniformly coat all sides with a phosphor and the whole frame will light up when electrons strike it. But to create a picture on a TV screen, we need a few regions to light up and a few regions to remain dark. That way we can see the landscape of lit regions as a single image. We also need the lit regions to be able to change quickly — so quickly that as the pictures change, our brains think it's a moving scene rather than a series of still images.
Enter: a major invention of the early 1900s, the cathode ray tube. A cathode ray tube creates a stream of electrons through the tube flowing towards the screen. Imagine electrons as a flock of birds flying in one direction towards a wall, which in this case is the screen. Now imagine a bird traffic signal manager that can direct birds towards different points on the wall. We similarly need a way to direct electrons to different points on the screen.
If we know how much to deflect them, and how fast they are moving, we can plan exactly the location on the screen they will strike. And where an electron strikes, the region will light up. Just like the conductor of an orchestra, if our bird traffic manager can direct birds to different locations on the wall, we can continuously change the parts of the screen that will light up, creating a moving picture.
Magnetic fields
Now, even as we have a stream of electrons, how do we deflect them at will? This is done with the help of magnetic fields. Electrons have a charge, and one can move charges using two kinds of forces. Electric fields can make them faster or slower. This is what we see in clocks, wires, and torchlights, where batteries create the fields. A magnetic field, however, can do something more interesting. It doesn't change the speed of charged particles but it can make them move in a circle. It's like when you tie a ball with a thread: you can pull the ball towards yourself, or you can try to swing the ball around.
This other kind of force is called the Lorentz force — and it is applied by magnetic fields.
We can use magnetic fields to move the electrons to the location we are interested in, and thus we have our traffic police. A bunch of copper wires and coils can be used to create these fields. Such electronic circuits are called analog.
While a lot of physics and engineering goes into creating the perfect images you see on TV, the basic physics is simple. We understand how electrons get directed to different locations on the screen. As they strike various locations, the phosphor lights up. As the TV signal changes the points where the electrons strike, the screen changes continuously, playing for us our favourite Bollywood film.
Boxes to screens to…?
With time of course, physicists discovered new concepts and we didn't need all those coils of wire to move electrons. In 1947, scientists at Bell Labs in the U.S. invented the transistor. This device led to the computer boom and eventually semiconductor electronics.
Here, too, the physics concepts are similar. Instead of phosphor, we have another light-emitting material called gallium-arsenide-phosphide (GaAsP), which throws out light when electrons go into them. And instead of rays of electrons, we can direct electrons more precisely using electronic motherboards like in our laptops. If you're wondering how these newer technologies work, that's a story for another day.
The reason we could make moving pictures was the magnetic field's ability to deflect electrons. Here the electrons were moving in three dimensions, the same number of dimensions we live in. The dimension of space is the number of directions in which we can move. For example, if something can move in all directions — right-left, front-back, top-down, it's said to exist in three dimensions.
A TV of the future may just take advantage of electrons forced to move in two directions: front-back and right-left, like an ant on a table. This happens in some particular materials that physicists can make in the lab.
It turns out there is a big difference in physics between two and three dimensions. In two dimensions, if temperatures are very low, a group of electrons can behave in a funny way. They form what is called a fractional quantum Hall state. Here, effectively new particles emerge that have just one-third of an electron's charge, and they can move only along the edges of the material. Robert Laughlin, Horst Ludwig Störmer, and Daniel Tsui won the physics Nobel Prize in 1998 for discovering such particles.
These kinds of particles are called anyons. They are completely different from the particles we usually encounter in three dimensions, like electrons and photons. Scientists are trying to build a new, powerful kind of quantum computer using anyons as their qubits. These machines could be responsible for bigger technological revolutions in future, and not just TVs.
But for now, we still don't understand all the physics of anyons. The Wolf Prize, one of the most prestigious physics prizes, was given to Jainendra Jain among others in 2025 for developing the basic understanding of some of this physics. Interestingly, Prof. Jain, who now lives in the U.S., did most of his early studies in India including in Maharaja College in Jaipur and at IIT Kanpur.
If you are inclined to understand some of the physics that goes on here, you'll need to learn quantum mechanics and condensed matter physics. You can consider taking a course in physics here in IIT Kanpur, where some of us teach.
Future TVs
Just like the invention in 1947 of transistors soon gave rise to the first TV, the discovery of particles with fractional charge may turn contemporary TVs into something we can't even imagine now. We never know how discoveries in quantum condensed matter physics today will change the world in the next 30 years.
But just like the warmth of a hot tea on a monsoon evening, the charms of Bollywood classics and basic physics never get old.
The next time you watch an emotional scene unfolding on your TV, don't forget to thank the electrons and the magic materials working away behind the screens.
Adhip Agarwala is an assistant professor of physics at IIT Kanpur.