EM Wave Reflection: Why Parabolic Reflectors Differ from Mirrors

In summary, satellite antennas are coated with paint and have a rough surface, causing light to scatter in many directions instead of being reflected like a mirror. Spoons, on the other hand, have a smoother surface and are better at reflecting light due to specular reflection. The microwaves used for communication are not affected because their wavelength is much larger than the surface roughness. This difference in behavior is also seen in other types of electromagnetic radiation. The name for this phenomenon is the index of refraction plus the Fresnel equations, although terminology may vary.
  • #1
oneamp
219
0
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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  • #2
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)
 
  • #3
I suppose my question is, if the dish is scattering light, why is it not scattering the microwaves used for communication?
 
  • #4
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.
 
  • #5
Thank you. What is the name for this... 'reflective coefficient at a given frequency'?
 
  • #6
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.
 
  • #7
Thank you
 

1. Why do parabolic reflectors differ from mirrors?

Parabolic reflectors differ from mirrors because they have a curved shape, whereas mirrors are flat. This curved shape allows parabolic reflectors to focus and concentrate electromagnetic (EM) waves, while mirrors simply reflect them.

2. How do parabolic reflectors work?

Parabolic reflectors work by reflecting and redirecting EM waves towards a single focal point. The curved shape of the reflector causes the waves to bounce off at different angles, ultimately converging at the focal point.

3. What is the advantage of using a parabolic reflector over a mirror?

The advantage of using a parabolic reflector over a mirror is the ability to concentrate and focus EM waves. This makes them useful for applications such as satellite dishes, telescope mirrors, and wireless communication antennas.

4. Can parabolic reflectors be used for all types of EM waves?

No, parabolic reflectors are most effective for radio waves, microwaves, and other long-wavelength EM waves. They are not as effective for shorter wavelength waves such as visible light or X-rays.

5. How are parabolic reflectors and mirrors similar?

Parabolic reflectors and mirrors both reflect EM waves. However, mirrors are flat and reflect waves at various angles, while parabolic reflectors have a curved shape and focus waves to a single point.

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