"if a source of electromagnetic energy is kept on the focus of parobloid and the parabloid is a reflecting suf

Question

surface,the waves get reflected and travel parallel in to the air" i cant understand that ,does the source kept (on a dish antenna which is a parabola) transmit the waves back on to the dish surface first ,which are then reflected into the air for transmission,or if u cant understandmy question ,plz tell me the working of dish antenna which is parabolic in shape

Answer 1

I think you're quoting from an engineering textbook.
So I'm guessing you need an engineering explanation.

Basically the explanation borrows geometric optics principles from physics.

The source, located at the focal point of the dish, radiates electromagnetic waves towards the dish. Then, by geometric optical theory, the rays are reflected parallel to the axis of the dish.

A deep understanding of optical physics isn't terribly important but here is a link to help you visualise what's going on:
http://dev.physicslab.org/Document.aspx?doctype=3&filename=GeometricOptics_SphericalMirrors.xml

Look only at case 1 (under the special cases section).

Note that the diagram actually shows rays going parallel to the axis and then passing through the focus. This is the opposite to your context (though equivalent), where rays originate from the focus and pass parallel to the axis.

Answer 2

Yes, that's pretty much correct. The standard example is a car's headlights: the light is at the focus of the paraboloid and the rays travel from the light to the mirror and are reflected in parallel directions.

A dish antenna is more normally used for reception, which is exactly the other way around, but when used for transmission would also behave in this way, as you described it.

Answer 3

The waves originating from the source expand out spherically. Since they are from the focus of a paraboloid, as the waves expand they will encounter surfaces which are angled such that the reflected waves will form plane waves rather than spherical ones. So, yes, the waves reflect off the paraboloid IN ORDER to be focused into a plane wave.

Not all of the waves from the source are changed to plane waves, of course; if some waves are emitted directly forward they will not encounter the paraboloid. But since they are spherical waves, they will diffuse quickly while the plane waves will remain concentrated into a beam.