Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Radio wave transmittance
Name: N/A
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: N/A


Question:
Why do radio waves pass through most solids but light does not?



Replies:
There are two reasons that I am aware of, maybe someone else can add to it.
1. For a medium or obstacle to block electromagnetic waves, it must have thickness larger than the wavelength of the waves and it must have some internal states that respond to energy hc/l, where h is the Planck's constant, c is the velocity of light and l is the wavelength of that radiation. It so happens that most atoms respond to optical wavelengths but the levels corresponding to radio waves do not exist or are not populated.
2. You can also have free electrons or a plasma which reflects electromagnetic waves - as the ionosphere reflects waves with wavelength around 50 metres. None of these two are satisfied by normal obstacles on the Earth so . . .

Jasjeet


When any radiation enters any material it is partially transmitted and partially absorbed. As it passes through each layer of the substance a certain percentage of the radiation is absorbed. How big this "tax" on the radiation strength is determines how many layers the radiation can make it through before it "goes broke." Anything if thin enough will be transparent to any radiation; conversely anything thick enough will be opaque to any radiation. What determines the tax for radiation of different frequency is whether there is a place for the energy at that frequency to go, which is determined by whether there is many processes that can occur in the material at that frequency. It happens that the vibration of atoms in all solids is right about at the frequency of infrared light (heat), so the "tax" for heat radiation is high, and an umbrella blocks heat radiation from the Sun quite well. The electrons in many solids jump around at about the frequency of light radiation, so light can also pay a high tax and many things are hard to see through unless very thin. Radio waves are at low frequencies, so you need things that happen at low frequencies (slowly). Rotation of water molecules is one such thing, and so water absorbs microwave (radio) radiation, which is how microwave ovens do their thing. Otherwise there is not much, so the tax on radio is light. Electrons in metals can slosh back and forth in a piece of metal at slow frequencies, and so metals absorb radio at nearly all frequencies. But the lowest sloshing frequency is given by the size of the piece of metal, just as you can get "slower" waves in the tub than in the sink, so to *efficiently* absorb slow radio radiation like that of AM radio takes long pieces of metal, of which there are not many just lying around.

christopher grayce


Though of course radio waves do get absorbed by some solid objects, as anyone who has tried to listen to the radio while driving through a tunnel or behind a hill can testify (it may be blocking rather than absorbing). Also, there are such things as Faraday cages that are just big rooms covered with metal that can block most radio from the interior (the reason is that the metal surface tries to maintain a constant electric potential, making it hard for a fluctuating field to get through). The fundamental reason for the different behavior of different materials has to do with the way the electron quantum states in the solid are filled up. The electrons go in and fill all the states up to some energy, until there are no more electrons left. If the next state available for a new electron is at essentially the same energy as the last electron that went in (as is true for all metals) then the electrons behave as if they were "free" and block electric fields. If there is a "gap" in energy to the next available states then the electrons are not so free - they can only absorb, reflect, or otherwise tamper with electromagnetic radiation if it is at a frequency that is equivalent (using Planck's formula) to this energy gap. This is what leads to the different coloring of different materials - they all have different energy gaps. Metals are special, because they have no energy gap at all. Semiconductors are kind of in the middle -their gaps are generally pretty small, but usually too high to block radio.

Arthur Smith



Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory