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physics please help!!

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A satellite dish is designed to pick up radio waves from geostationary satellites about 36000 km about the Earth's surface.
If the dish's surface is a part of a sphere of radius R, where should the receiver be placed compared to the center of the dish?

1. At a distance R/2 from the center of the dish, behind the dish (on the convex side).

2. At the center of the sphere, a distance R from the dish.

3. At a distance R/2 from the center of the dish, in front of the dish (on the concave side).

4. At a distance R behind the dish.

  • physics please help!! -

    The satellte dish reflects the signal from the satellite to a detector in front of the concave side, at the focal point. It acts just like a mirror for light.

    You should know the forcal point of a mirror is at R/2.

    DirecTV satellite dishes are off-axis paraboloids, not part of a sphere. The focal point and detectors are thus kept out of the way of the incoming beam.

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