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Thread: Properties of Light

  1. #1 Properties of Light 
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    When people describe light they are usually not very specific about it. I've heard that it is an oscillating electric and magnetic fields that can act as a wave or a particle known as a photon. If I'm correct light travels in a straight line, so what exactly are the oscillations in the electric and magnetic field? Also I've heard that an electromagnetic wave is made of multiple photons. If this is true can somebody clarify?
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    Quote Originally Posted by Scheuerf
    When people describe light they are usually not very specific about it. I've heard that it is an oscillating electric and magnetic fields that can act as a wave or a particle known as a photon. If I'm correct light travels in a straight line, so what exactly are the oscillations in the electric and magnetic field? Also I've heard that an electromagnetic wave is made of multiple photons. If this is true can somebody clarify?
    An electromagnetic wave is an electromagnetic field which propagates. The electric and magnetic fields oscillate in the direction perpendicular to the direction of propagation. Quantum field theory (QFT) "quantizes" the electromagnetic field by which it is meant that it consists of photons. How exactly an EM Field is described in terms of photons is something you have to know QFT to understand and I've never studied QFT so I can't answer that question for you. However I myself think of it in terms of light being an electromagnetic wave and at the same time being a stream of particles called photons. That's pretty much how it's described in physics texts such as texts called Modern Physics or Quantum Mechanics. The field in terms of photons is found in texts entitled Quantum Field Theory. I hope this helps.
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    Quote Originally Posted by Scheuerf View Post
    When people describe light they are usually not very specific about it. I've heard that it is an oscillating electric and magnetic fields that can act as a wave or a particle known as a photon. If I'm correct light travels in a straight line, so what exactly are the oscillations in the electric and magnetic field? Also I've heard that an electromagnetic wave is made of multiple photons. If this is true can somebody clarify?
    It's a good question and the early experiments to find the photon ( Hanbury Brown & Twiss 1956) were doomed to failure as most natural light sources result in a wavefunction which is not an eigenstate of the photon number. In other words not a whole number of photons. It wasn't until 1986 that Granger, Roger and Aspect managed to devise an experiment that would isolate a single photon so it could be observed. I personally regard individual photons as little packets of EM disturbances. See page 24 onwards in this book.
    The Quantum Challenge: Modern Research on the Foundations of Quantum Mechanics - George Greenstein, Arthur Zajonc - Google Books
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    Quote Originally Posted by Scheuerf View Post
    When people describe light they are usually not very specific about it. I've heard that it is an oscillating electric and magnetic fields that can act as a wave or a particle known as a photon. If I'm correct light travels in a straight line, so what exactly are the oscillations in the electric and magnetic field? Also I've heard that an electromagnetic wave is made of multiple photons. If this is true can somebody clarify?
    I find the whole thing incredible! It is so marvelously complex and I don't think anyone really fully understands it. Light is a small spectrum of electromagnetic waves. I took a course to be an electronics tech and got a job doing do, but wound up doing most of my job measuring mostly the E-Field (electric) waves, but also the magnetic components of the waves as well. Whatever they say about Light in a vacuum, it does not act that way in a non vacuum environment. It bounces all over the place and it gets absorbed by some materials, and reflected by others, and I think (refracted?) by others.

    The speed, I just realized is different in different mediums as well. It may be all well and good to say that the speed of light by our made up measurements of distance and time is an Invariant, but in the world I know, these things pure magic in the ways they act. And we do know enough to use their magical properties to create lasers and TV and Radio and cellphone and over the air internet informational exchange as is exemplified on this forum.
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    It may be all well and good to say that the speed of light by our made up measurements of distance and time is an Invariant, but in the world I know, these things pure magic in the ways they act
    An interesting comment and I should like to focus on the words I have emboldened, if I may, mayflow, be so bold.

    The world you know, the world all homo sapiens knows is a macroscopic world (but not so large), in which we are subjected to roughly a 1g acceleration and live at a pressure of about 1,000 Millibars, traveling at relatively slow speeds, perceiving things through a very small window of the EM spectrum and over time periods that are a tiny fraction of the age of the universe, yet very much larger than the time two atoms take to form bond.

    And our intellect and instincts have evolved to function in such a milieu, to the extent that we have certain expectations of what constitutes normal behaviour - if I jump off a cliff I shall down, not up. Unfortunately it is all too easy to transfer these common sense expectations to the microscopic, or the nano-second event, or an excessively strong gravity field. But common sense does not work in these arenas and if we choose to continue to use it we will be seriously misled. The universe will ignore our difficulty and gaily proceed to behave as it has always behaved, our protestations not withstanding.

    Just a thought.
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    The classical physics of light is shown by Maxwell`s Equation, by James Clerk Maxwell in 1864. His brilliant mind united electric field and magnetic field forming electromagnetic field. An electric field creates magnetic field and vice versa. Then, the idea of quanta of light-photon is first described by Einstein in his 1905 paper, together with his Special Relativity and Brownian Motion explanation-thermal vibration. Basically, when light hits a metal surface, the photon of light will knock off the free electrons in the metal. This phenomena is observed by a change in frequency of the light reflected from the metal surface. If I recalled correctly, this is the principle used in solar cell, by exposing metal to sun. The discovery of light properties leads to various implications in Physics, creating the so-called Modern Physics, which in turn cuts into the very foundation of laser(Light Amplification by Simulated Emission of Radiation), radar, phone, fiber optics... In fact the satellite that orbits above your head contains an on-board clock calibrated according to Einstein`s General Theory of Relativity and Special Theory of Relativity.
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    Quote Originally Posted by Nicholas Kang
    Then, the idea of quanta of light-photon is first described by Einstein in his 1905 paper, ...
    Actually the first person to postulate the idea of light consisting of particles was due to Sir Isaac Newton. See Photon - Wikipedia, the free encyclopedia
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    Quote Originally Posted by Physicist View Post
    Actually the first person to postulate the idea of light consisting of particles was due to Sir Isaac Newton. See Photon - Wikipedia, the free encyclopedia
    I didn't see where the wiki article said that about Newton, and well the whole article seemed to not make a lot of sense to me.
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    Quote Originally Posted by mayflow View Post
    I didn't see where the wiki article said that about Newton, and well the whole article seemed to not make a lot of sense to me.
    You are right to be skeptical, since Newton was not the first to propose that light consisted of particles. That idea is far more ancient than Newton. It goes back to at least Aristotle, and it would not surprise me a bit if someone before him had expressed a similar opinion.

    But it was Newton who most prominently supported this position, so that is likely what confused Physicist into overstating Newton's priority.

    The short story is that Newton and Huygens each promoted different theories of light. Newton strongly supported a "corpuscular" (particle) viewpoint. Later, Young's experiments seemed to contradict Newton rather definitively. Then the photoelectric effect came along, seemingly explicable only in terms of a particle model. Einstein earned his Nobel for promoting what later would be called the photon as this light particle.

    The modern view is that the dichotomy of "particle or wave" is improper. Depending on what experiment you run, light can evince either a particle-like or wave-like behaviour, just as Italian can sometimes appear to be Spanish-like or French-like. Italian is its own language, so asking whether Italian is Spanish or French is asking the wrong question. So, too, is light its own "thing," neither purely particle nor purely wave.
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  10. #10  
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    Quote Originally Posted by mayflow
    I didn't see where the wiki article said that about Newton,
    You can either do a search in the page using the key word "Newton" or scroll down to where it says Historical development where it reads
    ...however, particle models remained dominant, chiefly due to the influence of Isaac Newton
    More of that history is found here Molecular Expressions Microscopy Primer: Physics of Light and Color - Light: Particle or a Wave? and here Luminiferous aether - Wikipedia, the free encyclopedia
    Isaac Newton contended that light was made up of numerous small particles.
    Quote Originally Posted by mayflow
    .. and well the whole article seemed to not make a lot of sense to me.
    I don't know how to help you in that respect. Perhaps we can take it on bit by bit. Suppose there is something specific you'd like to talk about? Then we can go from there?
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