Walker, Halliday & Resnick — Electromagnetic Waves: Problem 52

52 Project Seafarer was an ambitious program to construct an enormous antenna, buried underground on a site about 10 000 km\(^2\) in area. Its purpose was to transmit signals to submarines while they were deeply submerged. If the effective wavelength were \(2.0 \times 10^4\) Earth radii, what would be the (a) frequency and (b) period of the radiations emitted? Ordinarily, electromagnetic radiations do not penetrate very far into conductors such as seawater, and so normal signals cannot reach the submarines.