Question

A laser with wavelength λ is used for optical communication from a tower on land to a ship located a distance daway. The tower has a height of habove the water and the receiving antenna on the ship is at the same height. Unfortunately, the beam from the laser diverges so much that there is a reflection of the laser off the water as well as the direct radiation from the tower. When the two beams reach the ship they interfere destructively so that the ship cannot receive the message.

Ignoring any possible effects due to the curvature of the Earth, derive an equation that can be used to calculate how far the ship must move directly toward the tower to reach a place where the two signals constructively interfere; call this distance Δd. Y​our answer must be interms of only h, λ, and d.

Answer 1