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Light As a electromagnetic wave?

  1. May 19, 2012 #1
    Hi fellow members!
    I have a serious confusion for light being an electromagnetic wave? since the we know that electromagnetic wave is produced due to the motion of charged particle.Light reaches us from the sun due to nuclear fusion reaction but light is produced due to the heat or energy produced in fusion reaction. So there is no motion of charged particle. So, how light is electromagnetic wave?If motion of charged particle involves then which one is involved in motion? Also does EM waves also have photons?
    Answers will be highly appreciated!!
    Thxxxx.
    Faisal!!!!
     
  2. jcsd
  3. May 19, 2012 #2

    Jano L.

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    You are right that hot objects glow. For example, you can heat up a metal spoon in the flame of natural gas. This does not contradict electromagnetic explanation of the light though. The standard explanation is that the hotter the metal is, the faster are the vibrations of the atoms and electrons constituting it. Since these are charged particles, there is much of accelerated motion of charged particles going on in the metal and the particles produce electromagnetic waves.
     
  4. Sep 19, 2012 #3
    I learnt today on PF that the way I thought of photons was not "universally accepted". I thought the photon was surrounded by EM waves. I was told to "Just stick to the universally accepted idea that a photon is an amount of energy which is transferred when EM energy is emitted or absorbed by a 'system'. I'm a beginner and I hope it answers the question a bit.
     
  5. Sep 19, 2012 #4

    Drakkith

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    There are multiple ways of producing EM radiation. First, an accelerated charged particle will emit radiation during the acceleration. This explains things like radio antennas which move the electrons in the metal back and forth at the frequency of transmission.

    Second, you have atomic and molecular transitions. This is when an electron drops from a higher energy orbital to a lower energy one. This requires a release of energy which is given off as light.

    Third, you have nuclear transitions. These only happen at very very high energies, far into the Gamma range. Basically the protons or neutrons in the nucleus are in a higher energy state, usually as a result of fusion or fission, and when they drop to their ground states they emit gamma radiation.

    Practically all of the Sun's EM radiation is emitted as a result of "thermal radiation". Basically the particles in the Sun are moving around very quickly, which results in collisions and particles being accelerated, releasing radiation. The gamma rays released in the core are immediately absorbed, so they never make it out of the Sun. The energy is used to heat up the core, and as you get further away from the core the temperature drops until you get to the photosphere, which is around 5,000 kelvin. Here the Sun is finally transparent to EM radiation and thus the light emitted follows the laws for thermal radiation from an object of 5,000 k.

    As for photons, the best way I know of to think of it is this. Light is an EM wave that interacts in discrete packets of energy we label as photons. So it IS a wave, but if it wants to interact with something, perhaps to accelerate a charged particle, it can only transfer energy and momentum in small packets, aka Photons. The energy of each packet is dependent on the wavelength of the EM wave.
     
  6. Sep 20, 2012 #5
    In classical electrodynamics, there are three important quantities. There are the electrical charges, the electric fields and the magnetic fields. In any single inertial frame, one can exist without the others.
    Light is motion in the electric and magnetic fields. The charges don't have to be there. If an electric charge is there, then the fields can move it. The motion of the electric charges are needed to detect the fields, but they are not part of the fields.
    Thing of the earth as being connected to the sun by two ropes: a magnetic field line and an electric field line. Each end is tethered by a positive electric charge. One positive electric charge is on an ion in the sun. The other positive electric charge is on an ion in your retina.
    When the electric charge on the sun is moved, each line is snapped in orthogonal directions. The electric field is snapped in the direction of the electric charge, and the magnetic field is snapped perpendicular to the electric charge. The pulse going down the two ropes is the "light".
    When the pulse reaches the electric charge in your retina, the electric charge starts to move. A positive ion would move in the direction of the electric field, although the magnetic field would effect the motion a bit. The snapping of the electric charge starts chemical reactions that you see.
    Yep, photons don't fit comfortably in classical electrodynamics.
     
  7. Sep 20, 2012 #6

    nsaspook

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