1. Two towers built on hills are 50 km apart and the line joining those passes above a hill midway between them. what is the longest wavelength of radio waves which can be sent between the towers without serious diffraction effects caused by the central hill? 2. Is the width of the obstrucle in necessary? 3. I think the problem can be related to Fraunhofer diffraction for a small obstracle. The condition for this type of diffraction is R≥ a^2/λ. But this requires the maximum width of obstracle.