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Understanding Wave Nature of light

  1. Jul 9, 2004 #1
    I have a real confusion in understanding what they mean by wave nature of light. I have seen waves in amplitutde Vs time curves, ripples in ponds or the wave generated in a rope by shaking one of its end. How do I Imagine the motion of light in space?

    Does the amplitude/Intensity of the light raises and falls as it passes through space or
    Is it the path taken by light that is a wave ?
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  3. Jul 9, 2004 #2


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    Light as a wave is a changing relation beteen the electric and magnetic fields in the space the light passes through. Each of these fields varies periodically, and the Maxwell equations guarantee that where one is weak the other is strong. The variation makes the waves of light (and other waves like radio and microwaves, and x-rays, and so on).
    Last edited: Jul 9, 2004
  4. Jul 9, 2004 #3
    So you mean to say... it's the intensity of electric and magnetic fields that change with respect to time, and the 'wave' thing is a just a visual representation (Field Intensity Vs Time Curve)
    Last edited: Jul 9, 2004
  5. Jul 9, 2004 #4


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    In analogy to sound:
    Sound travels in waves. The "sound" does not vary in space (the air), but the air pressure does vary. It is the fact that the air pressure varies that gives rise to sound, but a variation in air pressure is not sound. That where frequency range comes into play. For example, a high pressure system can move in from the north, causing the air pressure to vary, but this only corresponds to a frequency on the order of 10-5 Hz. It so happens that the cillia in our ears do not stimulate the "hearing nerves" unless they experience variations in pressure above around 20 Hz. Then, the phenomenology of hearing, to which we associate the phenomenon of sound, is not the experience of these variations individually, but the experience of the stimulation of the "hearing nerves" that is selected by the particular cillia that resonate at the particular frequency. In other words, sound is not the individual pressure fronts; it is the aggregate of repeated variations in the pressure fronts within a certain range of frequencies. You do not hear a "thump" for every pressure front that impinges on your ear; you hear a "tone" which is essentially an interpretation by a somewhat indirect process.

    Similarly, light is a phenomenology that arrises out of the phenomenon of variations of electric and magnetic fields. The rods and cones in your eye respond to coupled variations in electric and magnetic fields within a certain frequency. The phenomenon of light is the aggregate of these variations travelling through space.

    A wave transfers energy. Sound is a transmission of sonic energy. The sonic energy exists because the material is disturbed from equilibrium. This causes a state of pressure in such a way that the material seeks to return to the state of equilibirum. However, due to inertia, the material overshoots the equilibrium point and causes the pressure to build up in the oposite fashion in such a way that the material again seeks to return to the equilibrium point that it overshot. The coupling in the material transfers this displacement to adjacent material. The speed at which this displacement gets transfered depends on the rigidity (or inversely on the elasticity) of the coupling. For instance, in air, the molecules are weakly coupled, and air can be squished and spread out somewhat easily, so sound has a moderately low speed (through the air). In a metal, sound travels several times faster than in air, because the material is coupled much more strongly (metal is much more rigid).

    Light is a transmission of electromagnetic energy. The electromagnetic energy exists because space itself is distrubed from equilibrium as manifested by the electric field (in the absence of a charged particle, space doesn't like to have an electric field). So space seeks to return to the state of no electric field, but to do so, it must change the electric field (from whatever value it has to zero). From Maxwell's equations, you can see that this induces a magnetic field. From intuition, you can see the magnetic field as a manifestation of some kind of "electromagnetic inertia" (magnetic fields don't like to change). So, space overshoots the equilibrium, zero-electric-field, state and keeps going, until the electric field builds up in the oposite direction. This process repeats periodically. Since differenct points in space are coupled by causality, the electric field disturbance of space is transfered at a finite speed: the speed of causality (a.k.a. the speed of light).
  6. Jul 9, 2004 #5
    Wave - Light Propagation
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