Question

A beam of electromagnetic waves of a wavelength 3.0cm is directed normally at a grid of metal rods, parallel to each other and arranged vertically about 2.0cm apart. Behind the grid is a receiver to detect the waves. It is found that when the grid is in this position, the receiver detects a strong signal but that when the grid is rotated in a vertical plane through 90 degrees, the detected signal strength falls to zero. What property of the wave gives rise to this effect? Account briefly in general terms for the effect described above.