The radio waves which carry information in a standard broadcast are an example of electromagnetic radiation. These waves are a disturbance, not of a material medium, but of electric and magnetic fields. When the wave is linearly polarized, the electric field points in a direction perpendicular to the propagation of the wave, although its magnitude varies, of course, in space and time. The magnetic field points in a direction perpendicular to the wave propagation and to the electric field, and the two fields propagate in phase.
The electromagnetic radiation is generated by an antenna, which is a wire or metal rod which points perpendicular to the direction of the intended wave propagation. An alternating current is generated in the antenna, whose frequency is the same as that of the radiation to be produced. The electric field of the resulting electromagnetic radiation points along the same axis as the current.
The electric field of the electromagnetic radiation encounters electrons on the receiving antenna, which is another wire or metal rod. The electric field creates a current along the receiving antenna. One way to have good transmission and reception is to have the length of the antenna be one quarter of the wavelength of the electromagnetic wave.
For the following questions, consider a situation in which a transmitting antenna points vertically, and the receiving antenna is directly to the north. The speed of light is 3 x 108 m/s.
If the alternating current in the transmitting antenna has a frequency of 107 Hz, what would be a reasonable length for an efficient antenna, according to the passage?