See Infrared With Phone Camera: Try It on a Remote

Hand a child a TV remote and ask whether anything comes out of the front when they press a button. They’ll say no. Then open the selfie camera, point the remote at it, and press. A bright violet dot flashes on the screen. You can see infrared with phone camera hardware because the silicon sensor inside responds to wavelengths your eyes have no receptor for, and a remote has been firing invisible light across your living room for years. The whole demonstration takes thirty seconds, and the follow-up experiments are better than the first reveal.

Key Takeaways

• You’ll make a TV remote’s invisible infrared LED visible on a phone screen, then test what blocks it.
• It teaches that detectors have ranges, that eyes are one detector among many, and that “invisible” depends on who is looking.
• Works from about age 7. The materials are a remote and a phone.
• What usually goes wrong: using the rear camera. Most phones filter infrared out of the rear camera and not the front one.
• Important correction: this is near-infrared, not thermal. A phone camera cannot see body heat, and the article explains why.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Infrared remote control	1–3 different ones	TV, air conditioner, set-top box, old DVD player	A cheap universal remote; any remote you point and click
Smartphone with a front camera	One, ideally two different models	Pocket	An old webcam plugged into a laptop works even better
Black plastic bin bag	One	Kitchen drawer	Black photographic film negative, or a dark plastic folder
Assorted test materials	Paper, card, aluminium foil, a drinking glass of water, a hand, thin fabric, cling film	Around the house	Any five opaque and semi-opaque things
A dim room	One	—	Close the curtains
Fresh batteries for the remote	As needed	Drawer	—

Time: prep 2 min, experiment 25 min, cleanup 0 min Cost: everything is already in your house Mess level: low

Do the Experiment: See Infrared With Phone Camera in Six Steps

Step 1 — Ask the question before you touch a camera

Does anything come out of the front of this remote when you press a button? Almost every child says no, and almost every child is wrong. Write the answer down so it can be checked later, because this is the prediction the whole activity hinges on.

Step 2 — Open the front camera and aim the remote into it

Switch the camera app to selfie mode, hold the remote 5 to 10 cm from the front camera lens with its emitter pointing at it, and press any button.

What you should see: a bright dot flashing on the screen, usually pale violet or white, sometimes pink. It flickers while the button is held. Press a different button and the flicker pattern changes.

Step 3 — Try the rear camera and compare

Same remote, same distance, rear camera.

What you should see: on most phones, nothing, or a much weaker dot. The difference between the two cameras on one phone is a manufacturing decision, not a fault, and it is the most interesting result of the first five minutes.

Step 4 — Test three different remotes

Try the television, the air conditioner and anything else you can find.

What you should see: all of them light up, with different flicker patterns and slightly different brightness. Some have two emitters. A remote that shows nothing usually has dead batteries, which is itself a useful diagnostic you can now perform.

Step 5 — Find out what infrared passes through

Hold each test material between the remote and the camera, and separately check whether the television still responds through it. Work through: a sheet of paper, black bin bag, aluminium foil, a hand, a glass of water, cling film, a thin T-shirt.

What you should see: the black bin bag is the headline. It is completely opaque to your eyes and the remote works straight through it. Paper blocks most of it. Aluminium foil blocks it entirely. Water absorbs it noticeably over a few centimetres. A hand blocks it. “Opaque” turns out to be a question, not a property.

Step 6 — Record the flicker in slow motion

Set the phone to its highest slow-motion frame rate and record the dot while someone holds a button down.

What you should see: bursts rather than a steady glow. The remote is sending coded packets, and in slow motion you can see the gaps between them. You cannot resolve the individual pulses, because the carrier runs at tens of thousands of cycles per second, far beyond any phone camera. Seeing the bursts and knowing you cannot see the pulses is the honest result.

What’s Actually Going On

A standard remote control emits near-infrared light at a wavelength of around 940 nanometres, a figure NASA states directly on its page about infrared waves (NASA Science). Human colour vision runs out somewhere around 700 nanometres, because the photopigments in our cone cells stop absorbing usefully beyond that. So the light is there and we have no hardware for it.

A phone camera’s sensor is made of silicon, and silicon has a band gap of about 1.11 electronvolts at room temperature (HyperPhysics, Georgia State University). A photon can only free a charge carrier in silicon if it carries at least that much energy, which corresponds to a wavelength of roughly 1,100 nanometres. Everything shorter than that, including 940-nanometre infrared, gets absorbed and turned into signal. Silicon is naturally a near-infrared detector, and camera designers have to work to stop it being one.

That is why your rear camera probably shows nothing. Manufacturers put an infrared-cut filter directly over the main sensor, because otherwise infrared leaking through the red, green and blue colour filters would wash out skin tones and make foliage look strange. Front cameras are cheaper, were historically lower priority, and often have a weaker filter or none at all. Hence the dot on one camera and not the other. The colour you see is meaningless: the sensor has no category for 940 nanometres, the infrared leaks unevenly through the three colour filters, and the phone’s processing assembles that into a false colour, which is why the dot looks violet-white rather than red.

Now the correction that matters most. Infrared is a wide band, and what your phone sees is only its short-wavelength edge. The region used for thermal imaging, where objects at everyday temperatures actually radiate, sits between about 8 and 15 micrometres, which NASA labels thermal infrared on the same page. That is roughly ten times longer than what a remote emits and far beyond silicon’s reach. A phone camera cannot see body heat, cannot find a draught round a window and cannot see in a genuinely dark room. Thermal cameras use entirely different sensor materials for exactly this reason. If a child concludes from this experiment that they now have heat vision, the experiment has taught them something false, so make the distinction out loud.

How to Explain It to Your Kid

Ages 5–8: Light your eyes don’t have a part for

Eyes are like ears. A dog hears whistles we cannot, not because the sound isn’t there but because our ears don’t have the right bits. The remote is shouting in a colour we don’t have eyes for, and the phone does. Have them hold the remote and press buttons while you watch the screen, then swap, so each of them sees it both ways. Then give them the black bag and let them discover that the TV still changes channel through something they cannot see through at all.

Ages 9–12: Wavelength as a number, and detectors as ranges

Give them the numbers. Visible light runs from about 400 to 700 nanometres. The remote is at 940. The phone’s sensor works up to about 1,100. Line those four numbers up and the whole result falls out of the arithmetic: 940 is outside the eye’s range and inside the camera’s. Then have them collect data. Make a table of materials with two columns, “camera sees the dot” and “TV responds”, and look for cases where the two disagree.

Ages 13+: Band gaps and why the filter exists

The band gap sets the cutoff. Silicon at 1.11 electronvolts responds to anything shorter than about 1,100 nanometres, so engineers add an infrared-cut filter to preserve colour accuracy. Ask them to work out the consequence: what would photographs look like with the filter removed? Then set the real investigation. Measure the dot’s brightness on screen at 5, 10, 20, 40 and 80 cm, using a fixed exposure and reading pixel values. The intensity should fall roughly with the square of the distance, and testing whether it really does is a proper experiment.

The question to ask: “Could you use a phone camera to find a hidden security camera at night? What about a hidden heating pipe in a wall?”

Why Teaching This Early Pays Off

The idea being installed here is that perception is instrumentation. Eyes, cameras, microphones and thermometers each have a range, and anything outside a given range is invisible to that device while remaining perfectly real. Children who hold that idea handle later topics more gracefully: ultraviolet and sunburn, radio and Wi-Fi, X-rays, ultrasound, the entire electromagnetic spectrum.

It also models something useful about wrong answers. The “my phone sees heat” conclusion is a reasonable inference from a real observation, and it is wrong. Treating it as productive raw material rather than an error to stamp out is the approach argued for in the learning-sciences literature on misconceptions, which holds that students’ incorrect ideas are usually built from genuine partial knowledge (Smith, diSessa & Roschelle, 1994). Saying “that’s a good inference, and here’s the measurement that rules it out” is a better move than “no”. A thirty-second demonstration will not change a trajectory, but the habit of asking what range a detector has is a tool a child can use for decades.

Before You Start: What Parents Should Know

Best age	7 and up. The band-gap version lands from about 13
Supervision	Minimal. A remote and a phone
Real hazards	None beyond normal care. A remote’s emitter is a few milliwatts and harmless, though there is no reason to stare into it at point-blank range
Don’t	Never substitute an infrared laser, an infrared illuminator from a security camera, or any high-power infrared source. Invisible light does not trigger the blink reflex, so the eye’s normal protection is absent and damage can happen without any sensation of brightness. A TV remote is the right tool and it is also completely safe

When It Doesn’t Work

• Neither camera shows anything. Change the remote’s batteries first. A remote with weak cells can still work at 30 cm while being too dim for a camera at 5 cm.
• The phone is too new. Recent flagship models often have good infrared-cut filters on both cameras, and some camera apps apply processing that suppresses the dot. Try the oldest phone in the house, or a USB webcam, which is usually the most reliable detector you own.
• The dot appears but your kid is unimpressed. Skip ahead to the black bin bag. Discovering that a remote works through a material you cannot see through is a bigger result than the dot, and it reframes the whole activity as an investigation.
• A remote shows nothing while others work. Some devices use radio rather than infrared, including many modern streaming boxes and games consoles. That is a correct and interesting negative result, not a failure. Point the remote away from the device and press a button: if it still works, it is radio.
• The slow-motion video looks like a steady glow. Your frame rate is too low to catch the gaps. Try again at the highest slow-motion setting, in a dim room, holding a button continuously rather than tapping.

Take It Further

Build a transmission curve: test ten materials and rank them from “passes infrared freely” to “blocks it completely”, then rank the same ten for visible light. The two orderings will not match, and the mismatches are where the physics is. Black plastic and water are the interesting cases.

Make the phone the transmitter: some Android phones include a genuine infrared blaster for controlling televisions. If you have one, install a remote-control app and point that phone at a second phone’s front camera. You have just built an invisible optical link between two devices, and sending a few deliberate on-off patterns down it makes a point about optical fibre that no diagram does.

See Infrared With Phone Camera, Then What? A Sequence

This one is a good opener, because it needs nothing and produces a result in thirty seconds. Follow it with how TV remotes actually work for the coding and protocol side, which answers the “why does it flicker in patterns” question this experiment raises but cannot settle. Then run the other end of the spectrum: fluorescence with tonic water under a UV lamp covers invisible light just beyond violet, and together the two bracket the visible band from both sides. A spectroscope made from a CD and a cereal box then gives the visible middle a measurable scale.

What to Watch For Over the Next 3 Months

• Week 4: they test a dead remote with the camera instead of asking you whether the batteries are flat.
• Month 2: they ask what else is around them that they cannot see, and Wi-Fi usually comes up.
• Month 3: they correct somebody, including possibly you, about the difference between a phone camera and a thermal camera. Let them.

Frequently Asked Questions

Can a phone camera really see heat?

No, and this is the most common wrong conclusion from this experiment. Phones detect near-infrared around 940 nanometres. Thermal imaging works between roughly 8 and 15 micrometres, about ten times longer, which silicon sensors cannot reach at all. Thermal cameras use different sensor materials entirely.

Why does the front camera work when the rear one doesn’t?

Manufacturers fit an infrared-cut filter over the main rear sensor so that colours stay accurate, since silicon responds to infrared and would otherwise distort skin tones and foliage. Front cameras historically got cheaper filters or none, so the infrared leaks through and reaches the sensor.

Is it safe to look at an infrared LED?

A remote control’s emitter runs at a few milliwatts and is harmless at normal distances. Infrared lasers and security-camera illuminators are a different matter, because invisible light does not trigger blinking, so the eye gets no warning. Use a remote and nothing stronger.

Why is the dot violet instead of red?

The sensor has no colour category for 940 nanometres. The infrared leaks unevenly through the red, green and blue filters over the pixels, and the phone’s processing turns that unbalanced signal into a false colour. The violet is an artefact of the camera, not a property of the light.

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. NASA Science. “Infrared Waves.” Introduction to the Electromagnetic Spectrum. https://science.nasa.gov/ems/07_infraredwaves/
2. NASA Science. “Visible Light.” Introduction to the Electromagnetic Spectrum. https://science.nasa.gov/ems/09_visiblelight/
3. Nave, R. “Semiconductor Band Gaps.” HyperPhysics, Georgia State University. http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/semgap.html
4. Smith, J. P., III, diSessa, A. A., & Roschelle, J. (1994). “Misconceptions Reconceived: A Constructivist Analysis of Knowledge in Transition.” Journal of the Learning Sciences, 3(2), 115–163. https://doi.org/10.1207/s15327809jls0302_1
5. Chi, M. T. H., & Wylie, R. (2014). “The ICAP Framework: Linking Cognitive Engagement to Active Learning Outcomes.” Educational Psychologist, 49(4), 219–243. https://doi.org/10.1080/00461520.2014.965823
6. National Research Council. (2012). A Framework for K-12 Science Education. National Academies Press. https://doi.org/10.17226/13165