Even though phones now provide an inexhaustible source of entertainment, going to a nearby movie theatre to watch a film still has its own charm. The shared loud laughter, the annoying phone calls of neighbours, and even the endless paan-masala ads featuring fitness-enthusiastic Bollywood actors remind us of simpler times when only humans were sometimes called intelligent.
But even though movie theatres have transformed from having inedible food to unaffordable food, a few things have changed for good. For example, one of the most magical things in recent multiplexes, even if you ignore the thousand-crore-movies and the ear-wrenching Dolby surround sound, is the humble-looking washbasins. Incidentally, when you place your hand below the tap, it becomes self-aware and offers you water.
In fact, multiplexes (and even airports and metro stations) now seem to be aware. Doors open automatically, paused escalators start moving, and with a wave of your hand, sanitisers, dryers, and soap dispensers activate.
So, have you ever wondered how a washbasin detects you? In fact, even before magical wands became popular, you had some in your homes. Each of your remotes — be it for TV or AC — works like one. You point it from a distance, press a button, and suddenly the TV or AC comes to life.
Hiding behind all of this non-Harry-Potter-universe magic is, as always, some beautiful piece of physics. And this time, it is of a unique type of invisible light.
What exactly is a wave?
Before we go on to the invisible, the first natural question to ask is what is light? And the short answer is — light is a wave.
A wave is a disturbance that moves periodically, like a child on a swing. Now imagine a large playground with a thousand swings, all tied together. If you push just one swing, the disturbance travels through all of them until even the last swing starts moving. This moving disturbance is what we call a wave. Please note that no kids have been harmed in conducting this thought experiment.
A wave has a frequency, which depends on how fast the swing moves back and forth. If it does it once in a second, we call it one Hertz. Instead, if it does it 1,000 times in a second, we call it 1 kilohertz.
We see waves all the time — in ripples on water when a stone is thrown, or in sound when a car suddenly honks behind us in traffic. Those are sound waves. But light waves are a bit strange.
The reason is simple. In water waves, it is the water that moves after getting disturbed. Similarly, in sound waves, it is the air that has been disturbed by that car (also may be your mental peace).
But what moves in a light wave? After all, sunlight travels from the sun to earth through outer space, which has nothing to move. Light moves through a vacuum.
Invisible light
Light belongs to the family of electromagnetic (EM) waves — a wave in which electric and magnetic fields oscillate. An electric field is generated by a battery, and a magnetic field by magnets. But here they are generated simultaneously and have to change with time. In nature, they can be generated in multiple ways, one of which is by electrons inside an atom when they jiggle close to the nucleus. The faster the electrons jiggle, more is the frequency of the EM wave they create.
When such waves fall on our eyes, we can see them — but only a very tiny window of such frequencies. These frequencies are between 400 and 800 terahertz. Just like a kilo-Hz means a jiggle 1,000 or 10^3 times in a second, a terahertz means 10^12 times in a second — really, really fast. These ranges of EM waves are termed visible light.
In all of physics, the study of light and its properties remains one of the beautiful fields of study. This area of study is optics.
So, continuing, even if we do not ‘see’ all frequency waves, it does not mean they are not present. In fact, we are drenched in electromagnetic waves of various types. Radio waves (our FM radio), cannot be seen, but our radio transistors can pick them up. Similarly, microwaves are another type of electromagnetic wave.
Even within the light we do see, the different colours — from red to blue — have slightly different frequencies. The frequency of red is slightly lower than that of blue. The kind of waves that give the remote magic are called Infrared (IR) waves. Here, “infra” means lower than, because their frequency is just below that of red light. These waves, although like visible light, are actually ‘invisible’. That is, our human eyes cannot see them.
Pick up a remote closest to you, maybe belonging to a (slightly older style) TV or an AC. If you see the front end, you may find a tiny bulb. This is actually an LED — light-emitting diode. It means it emits some light when switched on. Think of it like a torch. Now, if you press the on button, you may not see anything glowing and wonder if this LED is really working. Turns out, it is ‘actually glowing’, just that your eyes can’t see it. Many of our modern smartphone cameras are sensitive to it. The other day, some students here taught me that if you point your phone camera at the LED and press the on button on the remote, you may actually ‘see’ this IR light. Did you?
Well, in fact, if our eyes were sensitive to IR rays, this world would look very different to us. We could also look very different to each other since our bodies also create IR waves. If you want to see how we would look, you can go to a website called cool-cosmos. This is run by IPAC, a physics centre at the California Institute of Technology, created by physics teachers to learn about IR waves. They have an IR Zoo, which has pictures of many animals and how they would appear in IR.
Now, while you may agree that a remote creates IR waves, the TV or the AC needs to know about them. Here comes the second important actor — the sensor.
Sensors as switches
You may think of a sensor as a railway crossing manager. It allows the train to pass if it gets the correct torch signal from far. These sensors are called photodiodes. They conduct electricity if light falls on it. The torch is the IR-LED, the train is of electrons in any electrical circuit, and the crossing manager is the photodiode. When the remote is switched on (the LED is on), the IR light falls on the photodiode. The photodiode now allows for the current to pass and your TV or AC switches on. Otherwise, it is in the OFF state, and the current does not reach.
Just like for a train to pass, the manager needs to know which colour torch is the ‘green signal’. Similarly, the sensor is only sensitive to the IR signals of the specific remote. Otherwise, your TV remote could switch on the AC and vice versa. You may now wonder how one creates the specific sensor. That is a different story, but a marvelous amount of material science and engineering goes behind this.
The physics subject which deals with the “ why” behind all of this, is called condensed-matter-physics.
How washbasins detect hands
So what really happens in that multiplex washbasin?
Here, an IR LED is kept and a sensor is placed just next to it. It is like having a torch that you are holding and pointing it in a vast dark space. Since you are holding the torch and are pointing it in front, the light will not directly fall into your eye. But now imagine that suddenly a big mirror comes in front of you. That mirror will reflect the torchlight from your hands right into your own eye and you will be able to tell — “Aha! A mirror must have appeared!”
Something similar happens when you place your hand in the basin. Usually, the IR light coming from the LED does not fall on the sensor and the water is not running. But when you place your hand, your hand reflects the IR light and now it falls on the sensor. The sensor thinks of it like a “green signal” and switches on the circuit. This circuit is tied to water and gets the water running!
Quantum mechanics
You may still wonder, how does a sensor work? How do electrons know the IR wave? How does the LED work? Much of this requires learning of quantum mechanics, condensed matter physics, and optics, something which one is taught in physics courses, for example, here at IIT Kanpur. If you are interested, you should consider studying physics.
So next time, when you go to watch a movie in a multiplex, and find that the multi-crore movie did not stand up to your expectations — do not be disheartened. Enjoy the ads and ambience, and then take a break to use the washbasin.
Another movie would be at play for you — with ultra-modern sensors, quantum electrons, invisible IR light, and four centuries of physics just to create the perfect magical moment.
Adhip Agarwala is an assistant professor of Physics at IIT Kanpur.
Note: This article was updated at 4.10 pm on June 9, 2026, to note that 1 terahertz stands for 10^12 times per second.
