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Browsing on Wi-Fi, I can access stuff in my room even though my Wi-Fi has to go through a wall to reach me.

But how is this possible, when the Wi-Fi wavelength is 12 cm, as cited here?

AFAIK, for light, or any form of electromagnetic wave, to "bend" around walls, the wavelength must be bigger than the window size. So how can Wi-Fi bend around windows? And how can it penetrate walls?

As a broader question, when can electromagnetic waves penetrate and not penetrate walls?

The question is different from If both radio waves and gamma rays can travel through walls.

That question already presumes that radio waves and gamma rays can travel through walls.

I do not know whether light can "bend" or penetrate walls. Also, I question Wi-Fi signals, which I believe are in the infrared spectrum only.

Thomas Lee Abshier ND
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user4951
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1 Answers1

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The "bending" you are talking about is diffraction, and while 12cm wavelength is just about right to diffract around most everyday objects in your house, that is not the main propagation effect. Instead, there are two others. Most indoor stuff separating and thrown around in the spaces are either low permittivity dielectric stuff, such as bricks, gypsum, wood, etc., or metal posts, grill, railing, plates, etc. The dielectric stuff is usually dry and quite transparent at the lower microwave frequencies; there is some reflection at the interface but usually not much absorption. The metal stuff acts as an almost completely lossless mirror (check the grill in your microwave oven, its holes are much smaller than 12cm - the same wavelength as ~2.5GHz, but it is nearly a perfect mirror for that).

So to your specific question, there is no substantial diffraction as the microwave passes through the window, it does get a bit reflected from the pane and the frame, but mostly just passes through, same for the walls. Most of the problem of indoor propagation is in fact with the reflected waves that arrive almost simultaneously and with nearly the same amplitude as the primary through-wave (buzzword: Line of sight or LOS wave). Being coherent (i.e., same frequency and phase) these may interfere destructively, so the receiver may move to one particular location and instantly nothing. The instant is also important since the environment is not static, but rather changing.

For a building with reinforced concrete walls, if the wavelength is on the order of the separation of the steel bars, the EM wave with E-field polarization parallel with the bars will reflect instead of penetrating into the building. This effect is more common around 900MHz (another ISM and mobile phones) and 150-450MHz (mobile radio band).

Thomas Lee Abshier ND
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hyportnex
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  • That's why I don't get signals on my gsm phone I guess? – user4951 Apr 13 '17 at 18:59
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    @Jim_Thio GSM is ~900MHz but the reception depends on how far inside you are in the building. If you are close to a concrete wall that has the rebars then the field that penetrates via diffraction you can receive, there is always some that gets through, but deep inside you will have problems, diffraction is a very inefficient propagator. Try this in a COSTCO or Walmart store. Of course it gets also worse inside because of all the metal shelves that create the multiple reflections all around. GSM is not really spread spectrum, and it does not handle well multipath propagation (reflections). – hyportnex Apr 13 '17 at 19:11
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    I'm no physicist, but I think "microwave passes through the widow" has an unfortunate typo. –  Apr 13 '17 at 20:06
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    @PeterCooperJr. I think it passes through her too though. So not that much loss.:) Though maybe she does absorb. – DRF Apr 13 '17 at 20:08
  • @Peter_Cooper_Jr. yes, that is very unfortunate... – hyportnex Apr 13 '17 at 20:09
  • @hyportnex I am unfamiliar with the acronym "ISM", which you used in this sentence, This effect is more common around 900MHz (another ISM and mobile phones) and 150-450MHz (mobile radio band). I looked it up, and it seemed possible that ISM was an acronym for Industry Science and Medicine, but I still couldn't see how that definition applied to the sentence where you used it. Clarification, please. Thanks. – Thomas Lee Abshier ND Jul 06 '18 at 00:46
  • @Thomas_Lee_Abshier_ND ISM industrial, scientific, medical https://en.wikipedia.org/wiki/ISM_band – hyportnex Jul 06 '18 at 01:06
  • @hyportnex, Thanks, the link to the Wikipedia page. I think what you were intending to say in your sentence was: This reflection effect in steel reinforced buildings is strongly seen around 900MHz (which is a frequency authorized for ISM uses/devices and non-ISM communication uses, such as cellular phones) and also in the 150-450MHz (the business mobile radio band). Did I understand your intended meaning properly? – Thomas Lee Abshier ND Jul 08 '18 at 13:00
  • @Thomas_Lee_Abshier_ND yes, your wording is OK here but note that when you have edited my text you may have misunderstood the original meaning when I referred to "location and instant". Both location and instant of destructive interference are variable because the reflection environment itself is variable. Fading is time dependent, just listen to AM or HF radio and you will hear slow fluctuation of the amplitude even at a fixed location. The fluctuation is caused by reflections of the wave whose distance changes in time. At ISM the wavelength is shorter and the fluctuation is much faster. – hyportnex Jul 08 '18 at 16:25
  • @Thomas_Lee_Abshier_ND in the FM band (88-108MHz) the reason why you do not hear the fluctuation is because it is frequency modulated and information is in the frequency of the signal and not in its amplitude as in the various AM bands (500kHz-60MHz). There is an AGC in the AM radio to take out the slow amplitude variation and pass the faster ones where the information is but there is a limit to its intelligence, obviously. – hyportnex Jul 08 '18 at 16:29