Question

Q 34. 2

The laws of optics also apply to electromagnetic waves invisible to the eye. A satellite TV dish is used to detect radio waves coming from orbiting satellites. Why is a curved reflecting surface (a

Answer 1

In optics when a concave mirror or a concave polished surface is place in front of a beam of light ( beam of light means a group of rays' of light) the beam of light strike the polished surface or mirror and the meet a the focus, the focus is at the focal length(distance from the pole (center of the mirror or polished surface) ).

The condition that you must have learned is that when a source of light is at infinity distance from that concave mirror or concave surface, all the rays of light in the beam strike that mirror or surface and CONVERGE at the focus in front of the concave mirror/surface. Same is in case of electromagnetic rays. The source of electromagnetic rays is the satellite which is at infinity. This is why a concave surface is used, if a convex surface is used the rays of electromagnetic waves would have struck the surface but instead of CONVERGING at the convex surface's focus they would have diverted away from the convex surface.

And actual radio receiver is place on an arm that arm must have a length equal to the focal length of the reflecting surface. Why that length is equal to focal length? because the intensity or magnitude of electromagnetic rays CONVERGING at the focus(after getting reflected from the reflecting surface) is high in order to receive a better signal the arm is fixed at the pole and the actual radio receiver is at the focus at a focal length distance. All the electromagnetic rays in the beam of rays are CONCENTRATED at the focus.