If Radio Waves have a lower frequency than visible light, how can radio waves penetrate through surfaces that visible light can’t penetrate through?
In: Physics
Experiment Like I’m 5 version – have a go with sound waves!
Low frequency waves (low/bass notes) easily travel through walls, but the high frequency ones are absorbed easily by soft materials. Basically the same with electromagnetic waves 🙂
Think of a tiny ripple hitting a big pier. The tiny ripple will bounce off. Now think of a big wave hitting the same pier. It passes right through because it has a much bigger wavelength. The longer wavelength is why radio waves can penetrate stuff while light can’t.
The fact that visible light *can’t* penetrate most surfaces is what makes it visible.
Photons are emitted and absorbed by charged particles, especially electrons. When electrons absorb a photon, it moves them into a higher energy state around their atomic nuclei. However, electron energy states have discrete energy levels – think of them like floors in a tall building. You can’t be halfway between floor 5 and floor 6, you must be on one or the other. And, those energy levels can only have so many electrons in them. So not only does an electron have to move up an entire floor, if that floor is already full it would have to go all the up to the next empty floor.
If an electron can’t get all the way up to the next empty floor, it will not absorb the photon *at all*. It can’t absorb it and try and fail and then re-emit the photon, the electron just completely ignores the photon’s existence.
Since most things around us are made of molecules with many atoms and usually many different elements, the electrons are all bound up in very complicated ways. Which frequencies of light the molecule can absorb and which it can’t determines which frequencies get reflected off of it, which is what we perceive as color. And, in fact, our eyes rely on absorbing different frequencies in order to see – we have special pigment molecules in our retinas that absorb specific wavelengths. The higher the energy of the photon, the higher the “floor” it can send an electron to. And when electrons go back down, they emit a photon that has an energy level corresponding to how far the electron “fell”.
Photons with energy levels below visible light just don’t have enough energy to move electrons up even from the first “floor” to the second. Since the electrons can’t go up an entire floor, the radio waves are not absorbed and just pass right through. They still lose a little bit of energy because the radio waves still push on the electrons a little bit as waves rather than individual particles.
This is also what makes transparent things transparent. All of the electron “floors” in those molecules are full. The next empty floor is so high up that only very high energy photons are enough to move them. Photons with frequencies in the visible range don’t have enough energy, so they pass through. Usually, though, transparent things are good at absorbing other frequencies, especially infrared and ultraviolet. We just don’t notice that because they aren’t visible to us.
Hey! Photovoltaics lab scientist here. Light of any sort (radio, visible, xrays, etc) will keep moving until it interacts with something.
The thing is, light has funny rules for how it interacts with objects. Each object (really, each different type of material) basically has an “energy toll” that the light must “pay” in order to interact.
Radio waves, like you say, have much less energy than visible light (lower frequency = lower energy). In fact radio waves have so little energy, they can’t afford to pay the energy “toll” of any material out there.
Can’t pay the energy toll? Can’t interact. No interaction? You’ll never stop. So, radio waves are very good at traveling long distances without being stopped.
Well visible light can penetrate though those surfaces, if it is bright enough. Think of shining a flashlight through your hand or a piece of paper.
Radio waves are also blocked by solid objects, think of listening to the radio while driving through a tunnel.
So the difference is that because of the size of the wavelength, they interact with materials differently. Radio waves can pass through the walls of your house, because to those large waves, your walls are just like a sheet of paper.
TLDR; to put it very simply (and of course with great inaccuracies) Think of your walls like a screen door. Some things pass through, some things can bounce off, depending on their shape and size.
Based on your question, I think you’re imagining that something makes it ‘through’ a material because it sorta sneaks through holes you can’t see in the material based on its size.
The analogy I like best, although not scientific is the use of rain (the wall) and the difference between a fly and a car. Both of these are going to get smacked by the rain, but the car can travel through the rain just fine essentially because it’s so big but the fly isn’t going to even try because the rain would pummel it. This is somewhat counterintuitive relative to the intuition you’ve presented in your questions (which I infer, maybe rudely!) where you’re thinking the larger object is more likely to get “stopped”. This intuition is likely born in the idea that the wall is solid, but if you were light you’d see it as a torrential downpour of things your size, but if you’re a radio wave you see it as a torrential downpour of little annoying things.
When you go beyond ELI5 here you need to start talking about the actual structure of the wall and then things get complicated for the case of light – the actual structure of the electrons has a lot to do with what can and can’t get through.
It’s like having a big fork or something and spaghetti. You drop the spaghetti it just sits on the fork but if you take it easy and work it through the teeth of the fork it can pass right through. Less “frictional force” getting applied. More slow oscillation, takes it’s time to get around atoms.
Radiation does not phase a material by passing in between the tiny holes in it.
Radio waves have a lower frequency, and thus a higher wavelength. Remember that frequency is the number of waves per second and that the wavelength is the metric length of one wave.
Multiplying the two will, intuitively, give you the rate (or speed) at which the waves are produced, i.e., the speed of light.
So,
Frequency × Wavelength = speed of light = fixed value.
The speed of light doesn’t change. It is a natural constant. And so, if the frequency is lowered, the wavelength must increase in order to keep the value of light the same.
And as such, radio waves have a greater wavelength. They can pass through things that higher-frequency radiations cannot.
Think of those surfaces as color-tinted glass. They allow some frequencies to pass while blocking others. Except in this case, the “colors” are outside of the visible spectrum.
Different things are transparent to different wavelengths, with transparent things having a *window* of wavelengths that they are transparent to.
For example there are “[IR windows](https://jerseyir.com/infrared-windows-101-basics-need-know)” that transmit infrared light (are transparent in IR) but non-penetrable to visible light.
It has to do with the wavelength of the radiation vs the size of the things being interacted with. Mountains can block radio waves.
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