Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Radio vs. Light

  1. Mar 23, 2008 #1
    As I've been studying up on EM radiation, I've come to the conclusion that radio and light waves are not the same thing. Radio seems to be nothing other than a continuously flipping magnetic field (source: http://www.pbs.org/wgbh/aso/tryit/radio/radiowaves.html). In this way, there do not seem to be such things as "radio waves". Radio, in other words, is the same phenomenon as jiggling a magnet around a conductor in order to induce a current.

    Light propagation, however, is a totally different animal. Some people say that light is caused by electric currents feeding into magnetic fields (and vice versa), but this explanation doesn't make much sense. Aren't electric currents and magnetic fields just different sides of the same coin? In other words, they can't "cause" one another if there is no difference between the two. A single electric current will always describe a single magnetic field, and vice versa. So, if you increase the current or move the wire around, won't the field immediately adjust to it? This doesn't seem to be a formula for the indefinite propagation of a light wave!

    Light just seems to be a result of the enormous amount of pure kinetic energy that is released whenever a molecule or an atom is destroyed. I don't know what makes this kind of energy propagation "electromagnetic"!

    Am I the only one who thinks that scientists do a disservice when they lump together two vastly different concepts such as these under a single, amorphous umbrella called "electromagnetic radiation"?
  2. jcsd
  3. Mar 23, 2008 #2


    User Avatar
    Gold Member

    I don't know where to start....

    1] ...

    (OK, 0] Molecules and atoms aren't destroyed.)

    1] That energy that's released when a molecule is dissociated into atoms or smaller molecules can be released as likely in the radio frequencies as in the visible frequencies.

    Really. Light, radio, microwaves, X-rays and cosmic rays are all identical except for their frequencies.


    Different frequencies = different amounts of energy.

    Low energy radio waves and microwaves penetrate different materials than medium energy light waves, and are more difficult to focus. Each band in the EM continuum has different properties and behaviours. It just happens that we critters on Earth have evolved to be sensitive to a narrow range of them.
  4. Mar 23, 2008 #3
    I think you're taking the fact that there are various ways to produce electromagnetic waves and then making the wrong conclusion that the results can't be alike in that they are all e.m. waves.

    To make radio waves you might have some electrons repeatedly go up and down an antenna. To make visible light you might agitate atomic electrons into higher energy states, so they will emit light when they drop back down to lower states. Due to molecular vibration, all matter gives off some infra-red just because it's temperature is something above absolute zero. Gamma rays can come from a nuclear reaction that has left nuclei in an excited state and they emits energy when they drops to a lower energy state. You can make x-rays by having a beam of electrons hit a metal surface, and the energy they lose when the collisions decelerate them will be emitted as e.m. waves.

    Different frequency bands are produced by different causes. All of them, electromagnetic waves.
  5. Mar 23, 2008 #4


    User Avatar

    i'm repeating myself a little, but here is a simple way to look at it:

    imagine that you and i are standing some distance apart and facing each other. you're holding a positive charge and i am holding a negative charge and that we both are restricting our charges so they cannot move toward each other but they can move up and down and left and right (just not forward or backward). so i move my charge up a meter. since your charge is attracted to mine, your charge also wants to move up a meter and you allow that. then i move it down and your charge follows it down. now i move it to my right (your left) and your charge moves toward your left. then to my left (your right) and your charge follows it.

    now i move my charge up and down repeatedly and your charge follows it up and down. that is an electromagnetic wave that originated with me moving my charge around and that wave moved toward you (at the speed of propagation of E&M waves which is "c") and causes your charge to move correspondingly. in a very real sense, my moving charge is a "transmitting antenna" and your moving charge is a "receiving antenna". if, somehow, i could move my charge up and down a million times per second, you could tune your AM radio to 1000 kHz and hear a signal (a silent carrier). if i could move it up and down 100 million times per second, you could tune it in with your FM radio just between the 99.9 and 100.1 settings (provided no other stations were close by). if i could move it up and down 500 trillion times per second, you would see it as a blur of orange colored light. now i can't move it up and down an entire meter 500 trillion times per second because the speed of that movement would exceed c. but i can have a whole pile of like charges and move them up and down maybe 10 microns at a frequency of 500 trillion Hz. that is what happens in a transmitting antenna or something that emits visible light. charges are moving and that causes other charges to move. but they don't react instantaneously (as observed by a third party that is equi-distant to you and i).

    that's what the electromagnetic interaction is. (in a greatly simplified perspective.)
  6. Mar 24, 2008 #5
    From what I understand, the phenomenon of radio transmission requires that a conductor exists within the range of a definite magnetic field. So, the range of the magnetic field depends entirely upon the strength of the current that produces it. All that a receiver is measuring is the "flip rate" of the spatially constrained magnetic field. Because this field is of a definite size, there seems to be no such thing as a "radio wave", in the classical sense of an indefinitely propagating surge of energy. We might be talking about oscillations when we talk about the transmission of information through the flipping of a magnetic field, but we aren't truly talking about waves, are we?

    Light, as far as we can tell, exhibits true wave-like behavior because it propagates indefinitely through empty space where there is none of the atomic matter that allows for the phenomena related to electricity/magnetism. And, it is only through sensible, atomic matter that there can be electric currents/magnetic fields.

    I believe that Maxwell's explanation of how light waves propagate is wanting. It doesn't make much sense to say that light waves are really electric currents and magnetic fields quickly feeding into one another through empty space. Just like I said in the first post, an electric current and a magnetic field are two sides of the same coin. If you strengthen or move the current, the magnetic field adjusts to it immediately.

    I also don't mean to say that light energy is only created by "destroying" entire atoms. I think that anytime you fundamentally alter the character of an atomic configuration (such as a uranium atom splitting into two different atoms), the energies involved are of such greatness that they disturb the fabric of the spacetime continuum (or however you want to describe the fundamental "stuff" of the universe) in such a way that an ever expanding, transversely oscillating spherical shell is created.

    Does anyone really think that flipping a magnetic field ever leads to indefinitely expanding waves of energy? I've heard it said that television and radio signals are emitted into space in an ever expanding shell. I don't believe this to be true because magnetic fields don't seem to be "propagative". They just kind of sit there, do they not?

    My whole point is that these two phenomena seem to be fundamentally different in that one is an oscillation of a definite magnetic field and the other is an indefinite expansion of waves of energy that propagate through space itself. It seems that these differences are great enough for us not to lump them together under the generic blanket called "Electromagnetic Radiation".

    I just don't think that it helps anyone's understanding of the way that the universe fundamentally works by lumping together phenomena that are of such a vastly different nature as radio and light.
  7. Mar 24, 2008 #6
    First of all, note: it's not just a magnetic field flipping back and forth. It's an electric field and a magnetic field, both flipping back and forth, at right angles to each other.

    Yes, the TV signal radiates outward from the earth. If it didn't, how could an observer at some distance away be able to detect it, after a delay that is determined by the speed of light?

    Suppose you could track a given peak of a given cycle. At the transmitter, the peak happens right now. After a certain delay, the observer in space sees the peak happen. In this way, isn't the radio wave the same as sound? A loudspeaker produces high air pressure right now. After a certain delay, a plane of air with high pressure reaches the ear of the hearer.
  8. Mar 24, 2008 #7


    User Avatar
    Gold Member

    Uh. Time out.

    The whole point of scientific discovery is to progressively lump every thing we know into larger and larger lumps until we have only one lump. Only then will we understand how the universe works.
    Last edited: Mar 24, 2008
  9. Mar 24, 2008 #8
    This is the typical definition of the nature of light propagation. A radio signal, according to PBS, is nothing other than a magnetic field changing directions as the current in the transmitter goes one way and then another.

    So you think that a radio signal is an example of the indefinite propagation of energy rather than the oscillation of a spatially contained magnetic field? Sorry, I just don't believe it. Make me think otherwise.

    Say a receiving antennae is 50 miles from an transmitting antennea. I fully understand that the receiver will only register the flipping of the magnetic field after a certain delay. I'm just saying that the magnetic field itself does not propagate outward. Its spatial bounds are always fully determined by the strength of the magnet in question. This does not correlate to the phenomenon of sound. Sound waves will propagate indefinitely outward, just like light waves.

    Uh. Time on.

    I don't see how science does much good when it lumps things together that don't deserve to be lumped together. If we do, then we will never understand how the universe works.
  10. Mar 24, 2008 #9


    User Avatar
    Gold Member

    That is a brave admission. And an excellent step toward learning more.

    Would you have find the universe a more understandable place if the apple falling from the tree and the Moon going round the Earth had never been lumped together? If they were taught separately? Are those two events unrelated in your eyes? On the surface they sure seem unrelated...

    To the eyes of everyone before Newton, that was the case. Newton's realization that - despite surface appearances the single same phenomenon underlies both of them - is widely considered to be one of the greatest moments in the history and advancement of science.

    Have you ever heard of the three blind men describing an elephant? The first blind man describes an elephant as a fat, prehensile, boneless tentacle, the second decribes it as a huge, flat, wrinkly bulge covered with pockmarks, warts and hairs. The third describes it as a thin rope-like thing with a tuft on the end. Each blind man thinks the others are barking mad.

    Then along comes a sighted man and tells them they are all describing the same from different angles.

    It seems to me, you are one of the blind men (metaphorically of course :smile: ) telling the sighted man he's a fool.

    It is the commonalities, the links, the connections between phenomena, the patterns - the symmetries - that reveal the universe to us. The differences in phenomena merely shine a light on our areas of ignorance.

    As you study more, you will come to realize that radio and light have way more in common than they have distinctions. That the distinctions you are seeing are superficial, and that it's the similiarities that are deeply meaningful and enlightening.
    Last edited: Mar 24, 2008
  11. Mar 24, 2008 #10
    What is the frequency range of photons? Can electrons emit energy packets of radio wave frequency objects in a manner similar to how they emit photons? Do gamma rays have properties similar to light, such as photons?
  12. Mar 25, 2008 #11
    Joey, there's no such thing as a field being "spacially contained" or "spacially bound". You take any charged particle or object, or take that little magnet that's stuck to your refrigerator door, and just jiggle it back and forth a little bit, and then the effect on the electric and magnetic fields will travel outward at the speed of light, for billions of light years. By doing that you just transmitted a radio wave that will travel to the most distant galaxies, while being too weak to be detected by anyone.
  13. Mar 25, 2008 #12

    It's photons in the case of all electromagnetic waves of any frequency.

    But it's not useful to visualize an electromagnetic wave as being a streams of photons traveling through space. The photon is a property of that energy just when it's absorbed by something or emitted by something. So we say that the wave properties and the particle properties are complementary. For some types of experiments say that light is a wave, and for some types of experiments say that light is particle, but don't say both at the same time.

    For example, an x-ray or gamma ray can harm living things by ionizing the atoms in living tissue. One electron absorbs one photon and gets ejected from the atomic structure. I hope that electron wasn't part of an important bond in someone's DNA.

    An electron traveling through space could even emit a radio wave just by being slowed down a little bit. This is a called a "free-free transition", to distinguish it from a kind of change in energy state in which binding to an atom is involved, that is, a "bound-bound", "bound-free", or "free-bound" transition.
  14. Mar 25, 2008 #13


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    First of all, aren't you very worried that your primary source of info is a "PBS" website? What happened to E&M text? Where do you think people at PBS get their ideas from? In other words, you're getting an interpretation of the source.

    Secondly, since when does radio wave "...requires that a conductor exists within the range of a definite magnetic field.."? I create RF in our wavguides using klystrons, i.e. via dumping electrons! I use no such "conductor", or any antenna.

    Thirdly, "changing magnetic fields" create E fields. That's from Maxwell equations (where those PBS fellas learned from). The frequency or the rate of change determines the amplitude of the E fields. The rate of change of both the E and B fields determine the frequency of that EM radiation. It is the frequency of such radiation that determine the "names" that we have given to them.

    I don't see how science does any good either by defending something based on ignorance. The fact that we do understand how "the universe works" more and more means that we never take anything for granted. However, things can't be advanced if we simply do not understand what is already known, and this is what you are doing. I don't think you can say how science is done simply based on your understanding from some PBS website, can you? Are you that confident of your knowledge of classical E&M that you can somehow make wholesale categorization of how science is practiced?

    In this thread, there appears to be no attempt by you at understanding where your knowledge of E&M is faulty. Rather, you somehow already have decided that scientists are wrong at the way it treats classical E&M (despite the fact that YOU are currently using devices that apply such knowledge). I would strongly advice you to review the https://www.physicsforums.com/showthread.php?t=5374" that you have agreed to. If you wish to understand and improve your knowledge of physics, this is one of the better places to do it. However, if you simply wish to exercise your ignorance, then this is the wrong place to be in.

    Last edited by a moderator: Apr 23, 2017
  15. Mar 25, 2008 #14


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Yes. I can pass a bunch of electrons through a structure, and it can create, depending on the structure, RF-range EM wave. Or I can pass bunches of electrons through a series of magnetic field and make it "wiggle" in its path. While this is not done normally (it's a waste of energy to do this at a synchrotron light source), it can be done in principle since such technique can easily generate EM radiation ranging from hard x-ray all the way down to IR.

    Depends on what you mean by "properties". We tend to associate properties with what we can measure, and what it can do. It is an EM wave, first an foremost, and thus, it satisfies Maxwell equations. There's no ambiguity about that whatsoever. All EM waves share that description.

    But what it can do can vary. Radio waves, for example, are simply to weak to cause ionizing radiation, whereas gamma rays can in certain dosage. Radio waves, due to its longer wavelengths, can go through a longer distances of air, but it cannot efficiently cause pair production, unlike gamma rays.

    But note that all of the "what it can do" here not really its characteristics in isolation, but rather how it interacts with something else (i.e. matter). So here, it also depends on the nature of that matter. While its EM description is true no matter if it is interacting or not, what it can do depends on what it is interacting with. We can see the visible range of EM radiation because that's what is causing our eyes to react that sends signal to our brain. There's nothing special about that EM range as far as the physics goes. There's also nothing special about the UV range, the IR range, the x-ray range, etc.. other than how they interact with matter that becomes useful for different purposes.

    Last edited: Mar 27, 2008
  16. Mar 25, 2008 #15


    User Avatar
    Gold Member

    Radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, gamma rays, cosmic rays. They are all describable in terms of photons of increasing frequency. (They are also describable as electromagnetic waves of decreasing wavelength.)


    Whether or not we study them as part of one group and whether or not we generate them or detect them with the same instruments, they are indeed all the same thing.
    Last edited: Mar 25, 2008
  17. Mar 25, 2008 #16
    Radio waves, microwaves, infrared, visible, ultraviolet, x-rays, gamma rays.

    For years I have been trying to find a cute sentence to serve as a mnemonic for the letters RMIVUXG. Does anyone here know of one?
  18. Mar 27, 2008 #17
    Surely after a number of years you would have memorised the electromagnetic spectrum in a similar method to how you memorised the alphabet?
    I think it just "makes sense" by considering the applications of the different frequency ranges (low frequency like radio/tv , then microwaves/radar.. IR.. vis.. UV.. (getting more exotic..) x-ray .. gamma.. cosmic..

    On topic: Thanks ZZ (and others) for their patience and good explanations
  19. Mar 27, 2008 #18
    I seek a mnemonic for it because I sometimes teach the subject and I have to force the poor kids to memorize it. The order "radio waves, microwaves, infrared, visible, ultraviolet, x-rays, gamma rays" is a "required concept" in the New York State regents physics syllabus.
  20. Mar 27, 2008 #19
    Cosmic rays are not part of the electromagnetic spectrum. That term is used to refer to high speed particles.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook