Recently I made this video. It can be seen and heard that the remote control (MTLogic) of our TV doesn't work when I place an object between it and the TV. This is the remote control:


It's obvious the signal doesn't reach the TV when the object is present. How can this be explained? I tried to do this at every angle (at the same distance though), horizontally as well as vertically, the device always directed towards the TV.
The object I placed first was a metal bottle. With a plastic bottle, the effect didn't occur just as it didn't with a glass bottle. Or aluminum foil, or a piece of paper. Toilet paper was the exception to the rule.

So the fact that the (hollow) object is made from metal is clearly of importance.

-Is the signal simply blocked (even though the object is made out of metal)?

-Does the object cause a disturbance?

-Is the signal reflected?

What's the answer? I guess the IR radiation is simply absorbed or reflected by the metal. Which might be the case for the other materials too?

Is it possible to do an experiment to show how much the IR beam diverges? E.g. by measuring the distance for which the remote control works again (in combination with the diameter of the object placed in the way)? What would be he associated formula?

Just for fun (thanx to a comment), I made a little video of the IR radiation coming from my remote control. I don't understand why it's white though. But that's a technical question, not suited for this site.

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    $\begingroup$ Is this different from a standard television remote control? Most of those have infrared LEDs which you can detect using a low-end digital camera or cellphone camera (though some higher-end cameras filter the infrared out again). I can detect the LED blinking in two television remotes using a cheap LG phone camera, but not using an iPad camera (and of course not with my eyes). If you can capture the LED on video you can experiment with which objects do or don't obstruct the beam. $\endgroup$ – rob Sep 17 '20 at 5:07
  • $\begingroup$ @rob It's an infrared remote control indeed. $\endgroup$ – Deschele Schilder Sep 17 '20 at 6:14
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    $\begingroup$ You could get creative and try to use two metal containers positioned in a way to reflect back to the tv. Or one container but from the other end. That way you can verify if it’s reflection $\endgroup$ – Superfast Jellyfish Sep 17 '20 at 6:46
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    $\begingroup$ To test a remote, try viewing the output through the camera on your phone. $\endgroup$ – R.W. Bird Sep 17 '20 at 14:38
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    $\begingroup$ @DescheleSchilder, Re, "...some higher-end cameras..." All color cameras block infrared. In order to make a color image, you need some sensors that see only "blue" light, some that see only "green" light, and some that see only "red" light. If your color camera dimly sees a glow when pointed directly at an infrared emitter, it's because the red, green, and blue filters are not perfect, and the emitter probably is much brighter than the camera is showing you. $\endgroup$ – Solomon Slow Sep 17 '20 at 16:41

A typical remote control uses an a small near-infrared light to communicate with the receiver. There are materials that are transparent to infrared light and opaque to visible light and vice-versa, but near-infrared is very similar in wavelength to visible red light, so for the most part the remote control will not work if something you can't see through is in the way.

  • $\begingroup$ Very true indeed. I tried thin aluminum-foils, glass with a high length of water, plastics, etc. The only exception to the rule was a piece of toilet paper or kitchen paper. But they have very small (non-visible) holes in it so the IR light can pass through. It's a challenge to find a material for which the rule doesn't hold! $\endgroup$ – Deschele Schilder Sep 17 '20 at 8:51
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    $\begingroup$ @DescheleSchilder The paper examples point out something important: there doesn't need to be a truly clear path from the transmitter to the receiver. Diffuse paths work. If holding a flashlight up to the material causes the other side to glow, there's a good chance the IR signal will be able to get through. We modulate that signal in a way which is pretty darn resilient, so not much of the original light has to get through in order to change the channel! I've bounced IR signals off of walls in this way. $\endgroup$ – Cort Ammon Sep 17 '20 at 8:54
  • $\begingroup$ @CortAmmon It worked for me too! $\endgroup$ – Deschele Schilder Sep 17 '20 at 10:08
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    $\begingroup$ My experience, in a small room with white walls, fresh batteries in the remote, and no sunlight coming in through the windows; is that I can point the remote pretty much any direction and the TV still detects and decodes the signal. As the batteries in the remote get weaker, the path from the remote to the TV starts to matter more. $\endgroup$ – Solomon Slow Sep 17 '20 at 16:43

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