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 Wave Behavior of Light

  1. Jan 9, 2017 #1
    Hi, I want to ask about the Wave behavior of light.

    Fine, I know all about the slit experiments and the resultant interference patterns - but I want to ask about the physical context.

    So light is the carrier of electromagnetic force, and thus its wave oscillation pattern represents an oscillation in the level of electromagnetism or electromagnetic force as it travels through space.

    When we look at a crest in the wave function, this is supposed to represent a peak in electromagnetism or consequent electromagnetic force experienced at that point. Meanwhile wherever the wave function approaches a centerline value, that represents a zero amount of electromagnetism or consequent electromagnetic force experienced. But when we look at a trough, what is that supposed to represent - force in the opposition direction? Is it negative electromagnetism?

    Sure, we know from the slit experiments that the troughs cause the dark parts of the interference patterns - but what are they, physically? Are they traveling "darkness"? We know from basic constructive/destructive interference that a trough can cancel with a crest to produce a zero/ambient result. So is the trough "anti-light" and "anti-force" value?

    The problem with purely relying on these slit experiments and their interference patterns, is that it's like explaining something using metaphors.

    I can easily see what a peak and trough represent in water waves. The peak/crest of the water wave is its peak height, while the crest is its lowest height which lies below the ambient "sea level". Height is very intuitive, and needs no metaphor to rely upon to be understood.

    But what exactly do the peaks and troughs and centerlines of wavefunctions for light mean?
  2. jcsd
  3. Jan 9, 2017 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Last edited by a moderator: May 8, 2017
  4. Jan 9, 2017 #3


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Both electricity and magnetism have + and - directions. The peak of a wave would be one direction and the trough would be the other.
    Those are the positions where the + and - directions of the wave from the two slots cancel each other. Those positions do not move.
    The experiment was done to prove the wave theory, not to explain waves. Take any wave that you are familiar with, like water, and you can do the same type of experiment. You will see interference patterns. I wouldn't use those patterns to explain or understand the wave. They are just consequences of the wave and they can be used as evidence that a wave exists.
    Last edited: Jan 9, 2017
  5. Jan 9, 2017 #4


    User Avatar

    Staff: Mentor

    In classical electromagnetism EM waves (which includes light) is an oscillation in the field vectors of the EM field. In other words, the direction and magnitude of the force felt by a charged particle at a particular point in space will oscillate back and forth.

    If the crest represents a field vector pointing in a certain direction, then the trough represents that field vector now pointing in the opposite direction. Convention is that the EM field vector arrows point from positive to negative. So a positively charged particle will be accelerated in the direction of the arrow while a negatively charged particle will be accelerated in the opposite direction. So when the vectors oscillate the particles experience forces and accelerate in alternating directions as the wave passes.

    The dark parts of the interference pattern are caused by the interference between the waves, not by the troughs.

    No, the trough is just the field vector pointing in the opposite direction. When you add the field vectors from each wave, the resultant field vector is simply the sum of the two. If the two waves are equal in amplitude but perfectly out of phase, the vectors add up to zero.

    I'm not sure what you mean. The double slit experiment simply shows the interference pattern of a wave that impinges on two slits. There's nothing metaphoric about it.
  6. Jan 10, 2017 #5
    Hi, thank you all for taking the time to respond.

    Okay, fair enough, thanks. But so when 2 pluses or 2 minuses coincide and have constructive interference, then they become a "double plus" or "double minus" respectively - and it's where they destructively cancel out that we see the dark bands, right?

    When you say they don't move, I wasn't referring to the point in space as moving, I was referring to the wave pattern as moving or propagating. Whatever is causing that momentary crest/trough to happen at that point, seems to be moving to the next point and the next point, where it makes the same crest appear. Can we say that some packet of energy is moving? Is that what a photon is - just a crest-&-trough?

    Well, that's why I am asking about the physical context of what the wave means. I already see and know that it exists, I just want to know what is the reason for it, and why it behaves the way it does.
    When a wave happens in water, it is due to mechanical pressure change - that's what makes the crest rise, and the trough depress down.

    But what is happening to make the EM field rise or fall in the other direction? When it rises, why doesn't it stay like that? When it falls, why doesn't it stay like that? What is the underpinning or underlying phenomenon behind this wave behavior? Is this known?

    What is making the individual oscillation happen? When that charged particle is feeling the EM force at that position and instant, what is making the EM force increase?

    Alright - but do we know what causes the wave-oscillation behavior?

    You mean 2 troughs constructively combining together to make an even bigger trough, right? Sure, fine.

    Do the electric and magnetic components always oscillate in sync with each other? Can you ever have the magnetic part out of phase with the electric part?

    So really then, those who say "light is a wave" are perhaps overstating the case - it simply has some superficially wave-like behavior.
    Even water is composed of particles and yet shows wave-like behavior.

    I just wish I could understand what exactly is propagating in this wave-like way.
  7. Jan 10, 2017 #6


    User Avatar

    Staff: Mentor

    Unfortunately there isn't much to say as to the why light behaves like a wave. We know how it works, but we don't know the why much beyond what you've already been told.

    I wouldn't say that it's "superficial" behavior. It's an integral part of understanding light.

    You might be able to in certain materials, but I really don't know. I'm not that far along in my education yet.

    No, I mean that a trough and a crest add together to make zero amplitude.
  8. Jan 10, 2017 #7
    So the idea of a particle having a velocity is straightforward. Does a wave have a velocity? If so, then how is this velocity measured, if a wave function has an infinite span?
  9. Jan 10, 2017 #8


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Yes. A wave has a velocity. The peeks and valleys move in a direction. The rate at which they pass a give point determine its frequency.
  10. Jan 10, 2017 #9


    Staff: Mentor

    Yes, this is known as far as anything in science is known, meaning that we have a theory which matches all known evidence. Since this is in the classical physics forum, I will assume you are only interested in classical phenomena, so the appropriate theory is Maxwell's equations.

    Maxwell's equations can be written in many equivalent forms. Under the assumption that all fields come from charges and currents, then the most useful form for understanding may be Jefimenko's equations. It says that the magnetic vector potential is caused by currents and the electric potential is caused by charges. These potentials propagate at c from their sources and get smaller the further away they go.

    So the field (potential) rises or falls because of how the charges and currents that caused it changed. Given an appropriate configuration of charges and currents it can, in fact, rise and then stay like that.
  11. Jan 11, 2017 #10
    When we look at a wave function at the points where the electric and magnetic field values are zero - can we honestly say that light exists at those points?

    Is it possible that the light is coming into existence, going out of existence, coming into existence, going out of existence, etc, etc?

    When we say that light can also be visualized as photons (particles), then it implies discrete minimum values on properties.
    For the picture/diagram posted by ZapperZ above - are there minimum values on the amplitudes or something like that, which would correspond to a single photon?
  12. Jan 11, 2017 #11


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Sure. That point is defined by the light wave, so it doesn't make sense to say there is no light there. Although the waves are zero at that point, their rate of change are greatest there. The zero points are moving at the speed of light, whatever that speed is in the material or vacuum that it is going through.
    Now you are talking about a fixed point. For a fixed point, the light frequency and magnitude are defined by the time history of the waves through that point. It is not right to say that the light "comes and goes" because the light is defined by the whole pattern of the waves. Even though the wave magnitude is smallest at the zero points, it's rate of change is greatest there. In electromagnetism, rates of change are very important.
  13. Jan 11, 2017 #12
    Okay, and so what causes light to bounce off matter and change direction?

    When you're listening to your car radio, and you pass under a tunnel and lose the radio signal, what exactly causes this to happen?

    Also, once again referring to ZapperZ's light wave diagram above - we know that the period of this wavefunction has a length associated with it. But does that amplitude have any length associated with it, or is it just purely an indication of field strength at that point?

    Can we say that the brightness of the light is the same at all points along that wavefunction? Would it be reasonable to say that the brightness of the light is zero where the function value is zero?
  14. Jan 11, 2017 #13


    Staff: Mentor

    These are philosophical questions that depend strongly on how you define whether or not light exists at a point. You could probably come up with a suitable definition which would give you either answer you prefer.

    Quantum electrodynamics is considerably more complicated than Maxwell's equations. It is not simply a matter of truncating a classical plane wave. If you want to pursue that topic you should open a new, more focused, thread in the QM section. But until you have mastered Maxwell's equations it would probably be a waste of time.
  15. Jan 11, 2017 #14


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    attenuation of the radio signal caused by the absorption of the tunnel material ... rock, concrete etc
  16. Jan 11, 2017 #15
    it is a bot. https://twitter.com/smokingwheels
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted