EM Wave Reflection: Why Parabolic Reflectors Differ from Mirrors

[oneamp]

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

 

[Drakkith]

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)

 

[oneamp]

I suppose my question is, if the dish is scattering light, why is it not scattering the microwaves used for communication?

 

[UltrafastPED]

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.

 

[oneamp]

Thank you. What is the name for this... 'reflective coefficient at a given frequency'?

 

[UltrafastPED]

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.

 

[oneamp]

Thank you