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

[Desmostylus]

handy, Dr David, you might want to look at the other thread: TV remotes & mirrors

[jpalm]

All this talk about reflecting things is great, but I have a question that I still don’t have answered from the original article. If free electrons are required to reflect light, EM waves, whatever, then how can I even SEE things that, as well as I can remember, don’t have free electrons? Everything must reflect light. That’s how your eyes are able to detect things. Right? I know that my skin isn’t emitting light in the visible spectrum… it isn’t hot enough… it isn’t 5800 kelvin, like the Sun. But I can SEE it. So it’s reflecting ambient light. But it’s made up of a bunch of covalently bonded atoms with no truly free electrons. So what’s the deal? Give me the straight dope. (Oh, and by the way, MY mirrors are all made of glass, not metal. They might have a metallic coating behind them, but I can make a mirror that’s almost as good by putting a sheet of black paper behind a sheet of glass.)

[Irishman]

Mirrors are typically made by coating glass with a layer of metal on the backside. The metal does the reflecting, but the glass ensures the surface is smooth.

However, if you look close at your reflection, you will see a faint double image. This is the image of light reflecting off the front surface of the glass. When you put a black surface behind clear glass, this is the reflection you get.

I can’t respond to the rest of your question.

[Wikkit]

*Originally posted by WashU77 *
As time went by, the batteries got weaker, and so did the signal from the remote. When I stood by the bed, I eclipsed the interfering remote, and the proper one worked fine. When I layed down, the signal would once again interfere with the proper remote.

My DVD player has a green LED that flashes whenever it’s getting any IR signal, whether it’s from the remote for the DVD, VCR, or TV. Occasionally I’ll see it flashing away, and then get to play “Find the remote that’s being squashed.”

Took me a while to figure it out the first time. I ended up starting at the DVD player, covering the IR receiver, then tracking it back to it’s source in the manner of those moths that follow pheremone trails.

[jpalm]

Thanks Irishman. Anyone out there know why black paper placed against glass makes a good reflector, when neither material seems to reflect much of anything, one apparently transmitting most of the energy and the other absorbing most, and alluding to the original article, neither has all that many free electrons?

[John_W.Kennedy]

I believe the black just makes the rather dim reflection more visible.

[Chronos]

Right. When you look at an ordinary piece of glass, you see a very faint reflection, as well as a bunch of stuff on the other side of the glass. When you put the black paper behind it, you still see the same faint reflection, but without the stuff behind to distract you. Our eyes are very good at correcting for different light levels, so the faint reflection looks “normal”.

[chukhung]

One thing that I haven’t seen mentioned in this discussion are those very old television remotes that didn’t use infrared light at all, but instead used sound waves. There’s more information about these remotes here: http://inventors.about.com/library/inventors/blremotecontrols.htm. (Warning, annoying pop-ups.)

The improved “Zenith Space Command,” remote control went into commercial production in 1956. Zenith engineer, Dr. Robert Adler who based his invention on ultrasonics, designed the Space Command. Ultrasonic remote controls remained the dominant design for the next twenty-five years, and as the name suggests they worked using ultrasound waves. The Space Command transmitter used no batteries; inside the transmitter were four lightweight aluminum rods that emitted high-frequency sounds when struck at one end. Each rod was a different length to create a different sound that controlled a receiver unit built into the television.
…
Infrared devices replaced ultrasonic remote controls in the early 1980s.

I remember seeing these remotes in stores when I was a kid, but I never used one. (Being the youngest, I was the remote control in our household…)

How far did the sound carry? Could you “bounce” the sound off a suitable acoustic reflector? Did switching channels drive dogs crazy? Did it repel pests?

Anybody else remember those ultrasonic remotes?

[Desmostylus]

My parents had an ultrasonic remote. Not the kind described in your quote, but a battery operated one using a piezo transducer.

• The range was about 20-30 feet, but wasn’t line-of-sight. You could use it in the next room, as long as the door between the rooms was open.

• It seemed to be pretty much “omnidirectional”. You didn’t need to point it at the TV, but maybe that was a result of the sound bouncing off the walls and ceiling.

• The cats could hear it. One cat really didn’t like it at first, but got used to it. The other cat could obviously hear it, but didn’t care.

[Karen_Too]

I didn’t say you NEED free electrons, I said they were BETTER for reflection than un-free electrons. For example, metals with free electrons typically have reflectivity of 95-98%. Glass has a reflectivity of about 4%. I seem to recall (from all those sunscreen articles) grass has a reflectivity of 5%. Polished silicon (which is a semiconductor) has a reflectivity of 30%.

Cecil talks about ultrasonic remotes at http://www.straightdope.com/classics/a3_270.html

[Desmostylus]

*Originally posted by Karen Too *
**I didn’t say you NEED free electrons, I said they were BETTER for reflection than un-free electrons. For example, metals with free electrons typically have reflectivity of 95-98%. Glass has a reflectivity of about 4%. I seem to recall (from all those sunscreen articles) grass has a reflectivity of 5%. Polished silicon (which is a semiconductor) has a reflectivity of 30%.
**

What you actually said was:

Good conductors, like metals, make good reflectors. Metals typically reflect about 95% of the incident electromagnetic wave. But other materials also have electrons in them, so you can get (small) reflections from insulators, like glass.

In the other thread, TV remotes & mirrors I suggested that that wasn’t a good explanation for things (especially) like titanium dioxide, which aren’t conductors, and which are quite reflective.

Care to comment?

[Jeff_Lodoen]

*Originally posted by chukhung *
**Anybody else remember those ultrasonic remotes? **

We had a Quasar (Motorola) with that, but it was not ultrasonic. You could hear it. No batteries, two spring-loaded switches that pinged two metal rods. I took it apart (luckily I got it back together, too!).

Switch 1 turned it on, raised the volume thrice and turned it off. Switch 2 turned the VHF channel knob forward. =ping= kachunk =ping= kachunk =ping= kachunk

Oh, banging on a muted triangle would work in a pinch.

Anyone remember a ‘reset’ switch on the back of old TVs? Lighting would knock out the TV, then you pushed it and it came on. What was it doing?

[Irishman]

Circuit breaker?

[John_W.Kennedy]

The ancient “Space Command” remotes from the 50’s were also audible. However, the sensors in the television may or may not have detected ultrasonic harmonics.

[Wikkit]

The hammer mechanism made a noise as well as the rod. The part you heard was the hammer and all those bits; the part that actually sent the signal was the rod being hit.

The first experimental ones were at the very top of the maximum range of human hearing, so young people, especially women, could hear them. Supposedly really annoying.

[John_W.Kennedy]

Indeed, I was a child at the time, and had a healthy high range (our brand-new Scott/Garrand “Hi-Fi” came with a demo record that included a test track). I could hear a ringing up in that territory.