EM Wave Reflection: Why Parabolic Reflectors Differ from Mirrors

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SUMMARY

Parabolic reflectors, such as those used in satellite antennas, exhibit diffuse reflection due to their rough surfaces, which scatters light in multiple directions, unlike smooth optical mirrors that provide specular reflection. Spoons, being smoother, reflect light more effectively at the same angle of incidence. The discussion highlights that while the surface roughness affects light reflection, it does not significantly impact microwave reflection, as the wavelength of microwaves is much larger than the surface imperfections. The reflective coefficient at a given frequency is influenced by the index of refraction and can be analyzed using the Fresnel equations.

PREREQUISITES
  • Understanding of electromagnetic wave properties
  • Familiarity with the concepts of diffuse and specular reflection
  • Knowledge of the Fresnel equations
  • Basic grasp of the index of refraction
NEXT STEPS
  • Research the properties of electromagnetic waves and their interaction with materials
  • Study the Fresnel equations in detail for various materials
  • Explore the concept of reflective coefficients across different frequencies
  • Investigate the differences between microwave and visible light reflection
USEFUL FOR

Physicists, engineers, and anyone interested in the principles of wave reflection, particularly in the context of satellite communication and optical engineering.

oneamp
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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
 
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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)
 
I suppose my question is, if the dish is scattering light, why is it not scattering the microwaves used for communication?
 
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.
 
Thank you. What is the name for this... 'reflective coefficient at a given frequency'?
 
oneamp said:
Thank you. What is the name for this... 'reflective coefficient at a given frequency'?

In optics it is the index of refraction plus the Fresnel equations:
http://en.wikipedia.org/wiki/Fresnel_equations

The same principles apply to all electromagnetic radiation, but the terminology may vary.
 
Thank you
 

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