Electromagnetic waves are generated by accelerating electrical charges. It can be demonstrated using the experimental apparatus of Heinrich Hertz. The transmitter consists of an induction coil and two metal spheres separated by a narrow gap. The receiver is a loop of wire with a small gap. When a high voltage pulse is applied, the electric field between the spheres ionizes the air and generates a spark.The frequency of the spark depends on the natural frequency of the L-C circuit. The electric field oscillations create an electromagnetic wave. A spark is generated in the receiver at the resonant frequency. Replacing the receiver with a metal sheet, standing waves can be generated. The region between the transmitter and receiver contains a series of nodes and antinodes. The distance between two consecutive nodes and antinodes is half of the wavelength. So, the wavelength can be estimated. Knowing the wavelength and frequency, Hertz estimated the speed of the wave which was very close to the speed of light.