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Both light and radio waves are electromagnetic waves. That means they have almost similar properties. Both are EM waves, $E$ and $H$ fields are there in both waves... but My question is: "radio waves can pass through walls, but why can't a light pass through walls or opaque medium???"

Roger V.
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user93624
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It has to do with the frequency response of the materials in the wall. Different molecules absorb different frequencies (or wavelengths) producing an absorption curve called the materials spectral response. Lots of materials are very absorptive in the frequencies typical in visible light but start to open up (get clearer) in longer wavelengths. Generally the energy of the visible photons are higher than the RF photons so they can react with more stuff and get absorbed. The lower energy radio photons can't react so they pass thru. All bets are off with metal walls for both, but that's a different effect, reflection not absorption.

JRD
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  • To underline JRDs answer, radio frequency is the modulation of an EM radiation. The wavelength of the involved photons is typical infrared but it is possible to bring the antenna rod to glow and one get photons in the visible spectra of EM radiation. – HolgerFiedler Sep 24 '15 at 04:33
  • That is why the classification of radio waves https://en.wikipedia.org/wiki/Spectrum#/media/File:EM_Spectrum_Properties_edit.svg with the spectra from infrared to gamma rays is misleading some times. – HolgerFiedler Sep 24 '15 at 04:37
  • Some visible light is of course reflected from a wall, otherwise paint manufacturers would be out of business. – ProfRob Sep 24 '15 at 05:52
  • @HolgerFiedler Radio frequency is the frequency of the carrier wave and not of the modulation. – nasu Dec 13 '22 at 12:09
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While both radio waves and visible light are electromagnetic waves, their different wavelengths and the way they interact with matter explain why radio waves can penetrate walls to some extent, while visible light cannot. The longer wavelengths of radio waves allow them to diffract and pass through obstacles with less interaction, while visible light is more easily absorbed or scattered by solid materials, making it opaque to our eyes.

Darwin David
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One of the defining characteristics of an electromagnetic wave is its wavelength (which is related to its frequency). Radio waves have wavelengths ranging from 1 millimetre to 100 kilometres, while light has wavelength on the order of hundreds of nanometres.

Interaction between electromagnetic waves and objects can be roughly predicted with the relationship between wavelength and the object's size; if the wavelength is greater than the object's largest dimension, then the object is invisible to the wave. If the wavelength is smaller than the object's largest dimension, then the object is "visible" to the wave, and there will be interaction between the two (usually in the form of reflection and refraction). If the wavelength and the object's largest dimension are similar, then a complicated process called scattering or diffraction occurs.

Julio Nicolini
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    So why does visible light travel further through seawater than radio waves... – ProfRob Sep 23 '15 at 23:12
  • Propagation through seawater has losses due to seawater not being a perfect dielectric (not even approximately), and this loss is also frequency-dependent. It's another mechanism entirely. – Julio Nicolini Sep 24 '15 at 01:24
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    Partly true. So then how does your "mechanism" account for glass? – ProfRob Sep 24 '15 at 05:49
  • What especifically about glass do you mean? It is just another dielectric, and you can model it according to what you're interested in studying. At the most basic level, it just has a relative permitivity in relation to air. Also, my answer was about rough predictions for waves interacting with objects in a single medium, not about treating different media. :) – Julio Nicolini Sep 24 '15 at 08:03
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    Your answer suggests that as long as the wavelength of the radiation is on a par or smaller than the object's "size" (not clear what you mean by that), that it will be absorbed. What exactly in a wall is supposed to be comparable in size to the wavelength of visible light? Visible light does not have a wavelength on the order of nanometres, it is nearly a micron and is much larger than any atom/molecule in the wall. Even if this were the case, what then is the difference in your model between a standard wall and a glass wall? – ProfRob Sep 24 '15 at 08:16
  • Visible light has wavelenghts from about 400nm to about 700nm. Let's assume that the EM wave is propagating in free space. Then, when it comes into contact with a macroscopic object with largest dimension $L$, if $L \ll \lambda$ then it will be invisible to the wave, if $L \approx \lambda$ you will have to take diffraction into account and if $L \gg \lambda$ there will be large-scale interactions (reflection and refraction) depending on the object's material characteristics. – Julio Nicolini Sep 24 '15 at 08:31
  • So, what will your "model" predict for x-rays? Will they penetrate a body or not? – nasu Dec 13 '22 at 12:04