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EM wave reflection

  1. May 25, 2014 #1
    Why do parabolic reflectors for microwaves (e.g. satellite TV) not reflect light like a mirror?

    Why do things like spoons, which are presumably conductive, reflect light?

    Thank you
  2. jcsd
  3. May 25, 2014 #2


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    Staff: Mentor

    Satellite antennas are typically coated with paint and have a very rough surface, which scatters the light in many different directions (diffuse reflection) instead of being very smooth like an optical mirror is. Spoons are much more smooth and do a much better job at reflecting light. Much more of the light is reflected at the same angle of incidence that it hits the surface at. (specular reflection)
  4. May 25, 2014 #3
    I suppose my question is, if the dish is scattering light, why is it not scattering the microwaves used for communication?
  5. May 25, 2014 #4


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    If the surface roughness is small when compared to the wavelength the wave doesn't "see" it.

    Another factor is that different wavelengths react differently - many materials that are transparent to visible light (e.g., some clear plastics) are great reflectors in the infrared.

    Both effects could be used.
  6. May 25, 2014 #5
    Thank you. What is the name for this... 'reflective coefficient at a given frequency'?
  7. May 25, 2014 #6


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    In optics it is the index of refraction plus the Fresnel equations:

    The same principles apply to all electromagnetic radiation, but the terminology may vary.
  8. May 25, 2014 #7
    Thank you
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