Why does the remote work if I point it at the TV's reflection in a mirror? Plus: Why are there no clear metals?
Dear Straight Dope:
The signals from a television's remote control are invisible, yet one amazing thing is that instead of pointing the remote control at the sensor on the TV as normal, I can also point it at a MIRROR in front of the set (which is facing the sensor) and it will work! How can signals bounce off mirrors? I have not tried this with other remote controls, like say for an automatic car alarm or something but I guess that would work too.
Why wouldn't remote control signals bounce off a mirror? Light waves bounce (reflect) off a mirror; sound waves bounce (echo) off a mirror; even a ball will bounce off a mirror.
Maybe you have the idea that some kind of magic emanates from the remote, some kind of essence of vampire that is unsusceptible to reflection. Nope. Remote controls produce regular electromagnetic rays in the infrared band. Infrared is invisible to humans but it reflects off mirrors just like its electromagnetic neighbor "visible light."
Electromagnetic waves, which include visible light, infrared, ultraviolet, radio waves, x-rays, and microwaves, are special, oscillating combinations of electric fields and magnetic fields (hence the appellation "electromagnetic"). Mirrors tend to be metals, which have bunches of free electrons loafing around, amenable to conducting electric fields. When the electromagnetic wave hits the mirror surface, the oscillating electric field from the electromagnetic wave causes the metal's electrons to oscillate. And as you know from your physics lessons, a moving electric charge generates more electric and magnetic fields. The electric and magnetic fields generated by all the electrons at the surface of the mirror combine to create a reflected ray such that the angle of incidence equals the angle of reflection.
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 fact, when you really need some high reflectivity mirror (like in lasers) you use a stack of clear dielectrics, and basically play games with the transmitted portion of the electromagnetic wave to get it all to eventually reflect. To play these tricks, you need to either transmit or reflect virtually all of the light--metals tend to absorb some of light, which limits their reflectivity. Using dielectric stacks, you can get mirrors with 99.999% reflectivity.
Oooh, I just thought of something else that doesn't reflect--those turn-to-stone rays that Medusa gives off.
Other types of remotes reflect in the same way. Garage door openers use radio waves. I'm not sure about keyless car remotes, or car alarms, but they gotta be radio or infrared, right? Radio waves have long wavelengths. Where visible light is measured in nanometers, and infrared is measured in micrometers, radio wavelengths are measured in meters, so to get a really good reflection from radio waves, you'll need a large reflective surface to handle the long electric field oscillations. The garage door across the street reflects my garage opener rays OK. And perhaps you have seen shadow images on broadcast TV caused by reflections when airplanes fly overhead. (You old-timers will remember broadcast TV.) And you might start to understand why a long metal antenna improves TV/radio reception. (You old-timers will remember antennas.)
Dear Straight Dope:
Let me tell you that I've read my way through more than one workday at straightdope.com. So I figured there may be a little enlightenment to go around to satisfy a question I've had since about 10th grade chemistry: Why are there no clear metals? It would seem that sheets of transparent metal would be worth something to someone. I have faith. --ryan woodrum, FSU
SDSTAFF Karen replies:
From the above discussion it should be obvious that the very thing that gives metals their metal-ness, the free electrons, is also what makes the metals highly reflective. For a substance to be "clear" it has to transmit a lot of light instead of reflect it. Conversely, glass, which is a notoriously clear substance, is an insulator.