1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I Range of frequency of electromagnetic waves

  1. May 17, 2017 #1
    Theoretically speaking, does the frequency of em wave range from 0 to infinity?
     
  2. jcsd
  3. May 17, 2017 #2
    Well it can't be zero. It can approach zero, though. It also can't be infinity (being that infinity is not a real number). It can approach infinity, though.
     
  4. May 17, 2017 #3

    jedishrfu

    Staff: Mentor

    I think its limited by the Planck length when you go toward 0 and limited by the size of the universe as you go toward infinity.

    https://en.wikipedia.org/wiki/Electromagnetic_spectrum

    In classical physics of the 19th century, it was believed to be continuous going from 0 to infinity.
     
  5. May 17, 2017 #4

    anorlunda

    Staff: Mentor

    But Wikipedia on Plank length says that there is no proven physical significance of the Plank Length.

    Can we say that the lower (practical) limit on wavelength is the upper limit on energy? Whatever emits the photon must conserve energy.
     
  6. May 17, 2017 #5
    Do you mean that it's range is infinite from a whole numbers perspective? Because something's frequency can't literally be 'infinity'
     
  7. May 17, 2017 #6

    Drakkith

    User Avatar

    Staff: Mentor

    Whole numbers? No. Ignoring possible quantum and cosmological effects, the range includes all positive real numbers, whole or not.
     
  8. May 17, 2017 #7

    anorlunda

    Staff: Mentor

    It is more of a question of semantics. Neither zero nor infinite wavelengths are practically possible. But there is no theoretically defined 0<limit or limit<infinity.

    So I think the best way to say it is that the limits are practical, not theoretical.
     
  9. May 19, 2017 #8

    tech99

    User Avatar
    Gold Member

    If we uniformly accelerate an electron and then allow it to continue at constant velocity, the radiated E-field would seem to be unidirectional, and hence it must possess a zero frequency component.
     
  10. May 19, 2017 #9

    anorlunda

    Staff: Mentor

    That's interesting. Can you tell me the difference between a zero-frequency zero-energy photon and no photon at all?
     
  11. May 20, 2017 #10
    Does a wave exist with 0 frequency? If so, should it be just a wave pulse?
     
  12. May 20, 2017 #11

    Drakkith

    User Avatar

    Staff: Mentor

    No, a wave pulse consists of many frequencies that interfere with each other to form the pulse. A wave with zero frequency can't be called a wave at all because nothing is changing. There is no oscillation, no vibration, nothing.
     
  13. May 20, 2017 #12
    Based on a picture in book 'Fundamentals of physics', wave pulse does not look like as you said. Capture.PNG
     
  14. May 20, 2017 #13
    I know topic is Em Waves and brought a picture from mechanical waves. But way of imagining even an Em wave look like this only right?
     
  15. May 20, 2017 #14
    In electronics, a "zero frequency" signal would be considered DC. So for EM, would a stationary magnet be analogous?
     
  16. May 20, 2017 #15
    Yea... the first one sounds like a wave can exist with 0 frequency (case of DC) I don't know the second.
     
  17. May 20, 2017 #16

    Drakkith

    User Avatar

    Staff: Mentor

    Sure it does. The different frequencies interfere with each other such that they sum to zero or near zero everywhere outside of the pulse.

    You can probably think of it like that, but I would still say that a wave with zero frequency isn't a wave at all.
     
  18. May 20, 2017 #17
    I agree, DC isn't a wave at all, at least not by any definition I can think of, or even just common sense. I was just pointing out a convention, or thinking that I think I've seen, that zero hertz would be thought of as DC (but no longer a 'wave'). I'm pretty sure there is a software front end for a 'wave generator' that would let you set the "frequency" to zero, and apply a DC offset.

    Would a stationary magnet be analogous to that thought?
     
  19. May 20, 2017 #18

    Drakkith

    User Avatar

    Staff: Mentor

    To a DC current? I guess you could say they are analogous in the sense that there is no change in the "signal".
     
  20. May 20, 2017 #19
    Is the frequency in electric current is caused due its patterned flow in a conductor, like in AC current, energy flow half - cycle up and then half cycle down creating to and fro motion treating as wave but not the actual frequency at which electrons vibrate when the disturbance/energy flow through conductor... Am I right anywhere? And what's the difference between a signal and a wave?
    Thanks for clarification, so there is frequency even in wave pulse.
     
  21. May 21, 2017 #20

    Drakkith

    User Avatar

    Staff: Mentor

    The details of the electric current is a bit complicated. A simple explanation is that electrons are always whizzing about in all directions and current flow is the net flow of electrons in a direction. The frequency of this net flow is the rate of the oscillation in it. The electrons themselves aren't vibrating back and forth at this frequency.

    Well, I'd say that in the context of current flow, the signal is the measurement of the voltage or current flow at any particular moment in time, regardless of its properties. The behavior of the signal can be described as wave-like when it behaves a certain way, namely that there is a repeating pattern that a wave equation can be applied to.

    That's right. Mathematically, any pulse can be broken down into the waves composing it by using a Fourier Transform.
     
  22. May 21, 2017 #21
    Does that mean electrons literally displace and flow?
    The frequency of this net flow is the rate of the oscillation in it. The electrons themselves aren't vibrating back and forth at this frequency.[/QUOTE]
    Rate of oscillation in what? The whole bunch of flowing electrons? That whole bunch move back and forth as they flow just like a ship sailing back and forth in harsh waters? Then some people say frequency as cycles/sec. Can it be anywhere linked to this?
     
  23. May 21, 2017 #22
    Since particles in EM wave are too small and a wave of size of universe means each particle should show enormous displacement stably without disturbance, I think it is impossible.
    According to De Broglie, all such microscopic particles travel in wave pattern showing wave nature in motion. Definitely, such a particle cant travel the universe without possessing even some wave pattern.
    Hence frequency tending 0 and wavelength tending to ∞ in impossible. That's my idea.

    I think so person who began this thread would have got his answer, we are unnecessarily going off - topic and wasting time.
     
  24. May 21, 2017 #23

    Drakkith

    User Avatar

    Staff: Mentor

    The electrons in the conduction band of a conductor are constantly moving about within the conductor in random directions and velocities. The electric field of an AC voltage source merely gives this random motion a small net velocity. In other words, more electrons move move one way past a point in a wire over time than in the other direction leading to a net flow of current in the circuit.

    The net flow is the sum of all the different velocities and it is this net flow that oscillates in direction and magnitude. Any single electron is not oscillating back and forth.
     
  25. May 21, 2017 #24

    davenn

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    I have always understood that they are
    and to confirm my thoughts I had to go google searching and found at least 4 sites that confirm that the electron/charge IS oscillating back and forward at the freq of concern

    do you have something to the contrary ?


    Dave
     
  26. May 21, 2017 #25
    Thanks for help, Understood what you have said and satisfied.
    But when I was referring a book named 'Concepts of Physics' by HC. Verma (Published only within India) but still has World - Class standards.
    In it he told to imagine flowing electric current as a people standing in a queue at box office for movie tickets and just told, If the person behind the queue pushes the person in front of him, he falls on another and like that whole queue gets knocked down. Does he mean electrons wont flow but that disturbance flow?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted