Inquiring Minds

Physics Questions People Ask Fermilab

Hello,
My name is Mike P. and this is my question.

If radio & light waves are both properties of the electromagnetic spectrum then why can radio waves pass through walls but light cannot?

Thank You

Hello Mike P.,

PART 1.

Let me first make sure the terminology we use is right.

The words "electromagnetic spectrum" are used to name a group of waves. Not any kind of waves, ( not acoustic, not mechanical waves) but electromagnetic waves. These waves have in COMMON that they are originated by electric or magnetic processes. But they also DIFFER in something which is called WAVELENGTH. (Simply speaking, their size.)

Check out our pages of light on the following web page

http://www.fnal.gov/pub/inquiring/more/light/index.html

( As an analogy think about "senior class". It is a name for a bunch of kids going to the same school and being roughly about the same age. However, they are all different. The differ in their weight.) RADIO waves and LIGHT waves are both PART of the "ELECTROMAGNETIC SPECTRUM", just as say JUDY and JOHN are PART of the "SENIOR CLASS".

Got the analogy?

Great!

PART 2.

Ok, now let us take a look at your question. I decided to give you two answers. One intuitive and not very precise, but still demonstrating the idea, and a second one, more precise and scientific.

PART2A.

The first answer uses again an analogy:

radio waves corresponding to a boy
light waves corresponding to a mosquito
the wall corresponding to rain

The answer to your question is hidden in the comparison of the sizes of the above objects.

A boy can easily run when it is rainy. Right? But a mosquito will never fly when it is rainy. Why? Because the size of a mosquito is roughly the same as the size of a rain drop.

If a mosquito entered into the rain, the first few drops would knock the mosquito down to the mud. On the other hand, since the size of a boy is much much bigger than the size of a rain drop, it is easy for a boy to run on the street even if it is rainy.

Now I show you how to use the above example in the case of the waves and the wall. What do you compare in this case?

You compare the size of the waves and a typical size of atoms in the wall.

The size of the waves is characterized by their wavelength.

I am telling you, radio waves are huge waves, their wavelengths are much much bigger then the size of atoms in the wall. According to the above analogy, that is why they go easily through the wall. ( As a boy did in the rain.)

On the other side, light waves are very very small waves, their size ( wavelength ) is comparable to the size of atoms in the wall. And that is why they are not able to go through the wall. ( As a mosquito cannot fly when it is rainy.)

CONCLUSION1: The radio and light waves are part of the electromagnetic spectrum, but are very different. Radio waves are much bigger than light waves (in terms of their wavelength). Radio waves are bigger then the size of atoms in a wall, that is why they go through, while light is a small wave and cannot get through the wall.

Does this make sense to you?

Good!

PART2B.

Before I give you a more precise answer, let us examine what you said. You claim:

"Radio waves go through the wall and light does not."

WELL, THIS IS NOT NECESSARILY TRUE ALL THE TIME!!!

If the wall is made out of glass, LIGHT WILL go through it.

On the other hand, if the wall is made out of iron, the radio waves WILL NOT go through the wall!!! appears

Wow, things are starting to be complicated right?

PART2C.

This leads us to a more precise answer to your question than the one I gave you above in PART2A. The real key is hidden in the STRUCTURE of the WALL. It matters, what the wall is made from, what kinds of atoms and molecules are its constituents. Also it is very important HOW these atoms in the wall are tight together.

As you know, every atom has a shell of electrons. These electrons interact between each of other and also interact with the outside world. The properties of these electrons dictate, whether a certain kind of incoming electromagnetic wave will go through or will not.

Some materials have the electron structure such, that they to be transparent for light but not for ultraviolet radiation ( for example glass, you will never get sun burned behind a window). But you can safely listen radio in your room. Glass is transparent to radio waves.

Some other materials have a different electron structure of their atoms, so they are not transparent for light, but are transparent for radio waves. Let us say a brick wall.

Also, as I said, you can find materials ( conductors, such as gold, iron, silver) that are neither transparent for radio waves nor for light.

CONCLUSION2: The atomic structure, especially the properties of the electron shells of atoms in the wall dictate if that particular wall to be transparent or not for a certain type of electromagnetic wave.

Hope, my answer satisfies your curiosity. Please keep wondering and asking questions. That is how you will learn the most about our world.

Arnold Pompos
Graduate Student at Fermilab

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