TV remotes & mirrors

Why does the remote work if I point it at the TV’s reflection in a mirror?

  1. Keyless car remotes and alarms use radio, not infra-red. The background infra-red level from sunlight would swamp the output from a tiny infra-red LED. There’s an easy way to test whether it uses radio. Open the car doors while the key is still in your pocket. See? It still works. It’s radio.

  2. The frequencies used in keyless car remotes and alarms are in the 300-450 MHz range. Cite: Wireless Fits Keyless Entry Applications

  3. 300-450 MHz means a wavelength between two-thirds and one metre. (e.g. 3x10[sup]8[/sup] ms[sup]-1[/sup] / 450x10[sup]6[/sup] s[sup]-1[/sup] = 0.667 m).

  4. Modern garage door openers use the same frequencies, although some older ones may operate at lower frequencies.

  5. A long metal antenna might improve radio reception for AM transmissions, which have wavelengths around 300 metres, but wouldn’t work well for TV, which uses wavelengths between and 0.33 metres and 5.5 metres (i.e. frequencies 54 MHz and 890 MHz)

Further to point (5) above, look at a picture of a TV antenna. It isn’t a long piece of wire by any stretch of the imagination.

The tv remote signal will also bounce of my forehead, the dogs, etc and handily change the station. We don’t need a mirror.:slight_smile:

Welcome to The Straight Dope Message Boards, simbamo, glad to have you with us. The image of you bouncing the TV remove off your forehead is… droll. And, after you’ve been around here a bit, you’ll learn that the line is “We don’t need no steenkin’ mirrors.”

Ground breaking discoveries are made often in our institutions of higher learning. Unfortunately adult beverages are sometimes present.

I would like to add another option to simbamos comment. I routinely bounce my remote signal off the ceiling. Since I am usually lounging on the floor while watching the tube (that being the position most conducive to allowing the destroyed brain matter run neatly out my ears) and the angle to my cable box is such that the direct route is ineffective.

As to bouncing off the dog or children, a novel concept. What drugs to you use to get them to stand still long enough?

We don’t need no steenkin’ drugs. (See that? I’m hip.) Dem dawgs are guaranteed to flake out on the rug between the tv and fireplace precisely 3 hours after they eat dinner which coincides with our prime time favorites. You can set your watch by them. When they hit the floor, it’s time for 24.

Probably television.

<< Unfortunately adult beverages are sometimes present. >>


Good point. How literal of you. I suppose it would be fortunate at an institution of HIGHer learning.

You silly people. The question really is “What is good at reflecting infrared waves? radio? visible light?” etc.

The reflecting thing does NOT have to be as long/wide as the wave it’s reflecting - remember, a photon is traveling in a perpendicular direction to its wavelength. I have no idea how wide an object must be to intercept a photon, but I do know that individual atoms interact with light all the time.

In any case, a normal mirror is quite good at reflecting visible light. Almost anything nonmetallic will reflect radio waves - note the horrible “ghosts” that appear in TV and radio (although radio tends to futz right out real quick and become absolutely unintelligible)_ As for infrared remotes, that might be different.

Good start.

You can probably get a reflection from anything, it just won’t be a good reflection if the reflecting surface is small compared to the wavelength. Think about why the resolving power of an optical microscope is limited to features comparable in size to the wavelength of visible light. If you want to see smaller things, you have to use smaller wavelengths of light, or switch to short wavelength electrons.

Cite? I’d be especially interested to see something about the reflective properties of plastics or air.

I have a friend that used to be able to bounce the remote
control off of his forhead as well. He thought it iwas due to
the “plate” in his skull - the result of a nasty golfing accident.
He was quite fond of saying that although he could change the
channel, he found that he often changed his mind and didn’t
want to watch the new channel aftr all! Funny thing was that
though that he could only do it with the old remote
controls - circa the laste 70s/early 80s. The newer remotes
didn’t work that way. Any ideas as to why?

Upon a second reading, perhaps the entire paragraph that I quoted in the OP is ill-advised.

The problem is that garage door openers, keyless car remotes and alarms don’t use directional antennas. They use loop antennas that have radiation patterns like this.

They radiate uniformly in all directions in the plane of the antenna. If you are holding such a device horizontally, it will radiate horizontally in all directions. Whether you point one end of the opener at the receiver (your car or your garage door) or at the garage door across the street doesn’t make any difference. If you place your body between the opener and the car, that might make a difference, but that’s because you’ve interposed something that absorbs radio waves (your body).

It may be better to re-do the rest of the article in terms of infra-red photons bouncing off mirrors, and that way you can also explain photons bouncing off the titanium dioxide in the ceiling paint, or bouncing off the dog, neither of which are metallic, and both of which do reflect infra-red light.

P.S., regarding the conductivity of glass. You could also point out that it’s possible to coat glass with transparent conductors, and that this is one of the things that enable LCD displays to work.

Bump, because the Staff Report has just been published.

The length/width of the reflecting surface does matter. This is how polarizing gratings work. Think of a grating as a lot of closely spaced parallel metal wires. The component of the EM wave which is parallel to the wires is easily reflected. The component which is perpendicular to the wires is only very poorly reflected. Result: the reflected light is very highly polarized. (That is, all its EM waves are in oscillating in the same direction.)

I couldn’t work that into the original Staff Report. Or the stuff about a very open mesh being able to highly reflect microwaves.


Yep. The new remotes use infrared light. The older remotes used sound waves.

I still remember the first remote control TV I saw. It was in 1965. Of course I had to take the remote apart to see how it worked. It had two buttons, volume/on/off and channel. When you depressed the button, you heard a sound that I can best describe as “CHUNGGGGG!” This was the button activating a rod which struck a tuning fork at a particular frequency, which was picked up by the receiver on the TV and either changed the volume/turned the set on or off, or changed the channel.

Later models still used sound waves, but used miniature synthesizers and tiny speakers in the remote to accomplish the same thing. With the synthesizer, you could control a whole range of items on the TV due to more frequencies being available…

I remember on these TVs I could change the channel by jingling coins or keys. Drove my dad nuts until he figured out how I was doing it.

Can playing with a Slinky change the channels on your TV set?

A question then, if the human body absorbs radio waves how come my car remote, which does work while in my pocket, works from a greater distance if I press it to my chin than if I hold it our in front of me? A friend pointed this out to me before you go wondering why I hold car remotes to assorted body parts :slight_smile:

The human body is a wonderful antenna.

If you don’t believe it, find a very weak FM radio station either in your car or on a portable radio with an external antenna, then alternately hold and release the antenna while listening to the radio.

You’ll hear the station stronger when you’re touching the antenna - your body is literally becoming the antenna at that point.