Well not invisible but… pretty close. When you think about RF remotes, you have to realize that the signal they put out is part of the whole electromagnetic spectrum that includes light, X-rays, conventional broadcasting, cell phones, everything. Electromagnetic radiation is everywhere and it comes in many flavors. When we talk about the kind of radiation that’s useful for transmitting coded messages (because let’s face facts, that’s what we’re doing), we’re really talking about the frequencies that are best for radio, or Radio Frequencies. See what I did there? Maybe you didn’t know that RF stood for Radio Frequency.
What’s pretty obvious if you think about it is that electromagnetic radiation behaves differently if it’s at different wavelengths. For example, a very small part of that whole electromagnetic spectrum can be interpreted by our eyes as visible light. There’s nothing special about that little chunk except that we Earth creatures have, after billions of years, evolved to sense those frequencies better, probably as a survival technique.
On the other hand, there is a whole chunk of electromagnetic radiation that’s pretty hazardous to humans. Gamma rays, X-rays, this sort of stuff goes through our skin and causes damage to our internal organs. Ultraviolet radiation is radiation just past our ability to see and it’s pretty bad for you too. Infrared radiation is also just past our ability to see, but it has the effect of exciting the atoms all around us to generate heat.
Most radiation seems to be pretty harmless in small doses. RF radiation is used for everything from TVs to garage door openers to cell phones and so far there’s no evidence that it causes any damage to us in the very low power levels that we use it. Of course, there could be evidence tomorrow and then it’s hard to know what we’d do then.
But none of this really answers the question of whether or not walls are invisible to RF radiation, or why that might be true. Let’s get back to that.
FM radio waves are pretty big. A wave could be 10 feet long. On the other hand visible light waves are really small, like a thousandth of a millimeter. Every kind of electromagnetic radiation is distinguished by the size of its waves, which is why we use the term “wavelength.” So FM radio waves travel quite a bit differently than visible light, and that’s part of the equation.
The other part of the equation, and this is going to mess you up a little if you didn’t already know this, is that nothing is really “solid.” Some things are very dense, but that doesn’t mean they are solid. Looking at your average piece of wood or plaster, it’s not totally waterproof or light-proof for that matter. Shine enough light behind it and you’ll see it passing through. Dip it in water and it absorbs some. This is because the structure of wood, plaster, glass, stucco, in fact everything is mostly empty. if you look at that stuff under a powerful microscope you’ll find that it’s more like a grid made up mostly of open space.
So if you are willing to accept that the wall, the window, the framing of the house, whatever, that it’s all mostly open space, then you’ll understand that some things can pass through that open space and some can’t. Some sizes of waves will just slide through the open space and some will bounce back. It turns out that FM radio waves happen to do a good job of passing through the stuff we typically build houses out of. There’s no magic there, it’s just that when all this stuff was invented someone just sort of noticed that there was a range of frequencies that worked well for this purpose and that’s the ones that were used for radio.
Most houses tend to be built out of stuff that lets RF transmissions go right through, but things made of metal tend to slow down or stop transmissions. That’s why antennas are made out of metal. Unfortunately there can be metal in your house as well. There can be stucco mesh, metal conduits, wires, and appliances built into the walls, and all of those things are going to keep the signal from your RF remote from passing through. Actually, most everything does actually stop some RF transmissions from going through… for example a signal from outside typically loses half its strength going through the walls. That’s why it’s harder to get cell service inside.
When it comes to an RF remote, however, and its ability to shoot through drywall and wood, it does a fairly amazing job, almost as if those items are completely invisible. That’s what we mean when we say that.