Physics » Electromagnetic Waves » Maxwell’s Equations: Electromagnetic Waves Predicted and Observed

Hertz’s Observations

Hertz’s Observations

The German physicist Heinrich Hertz (1857–1894) was the first to generate and detect certain types of electromagnetic waves in the laboratory. Starting in 1887, he performed a series of experiments that not only confirmed the existence of electromagnetic waves, but also verified that they travel at the speed of light.

Hertz used an AC \(\text{RLC}\) (resistor-inductor-capacitor) circuit that resonates at a known frequency \({f}_{0}=\cfrac{1}{2\pi \sqrt{\text{LC}}}\) and connected it to a loop of wire as shown in this figure. High voltages induced across the gap in the loop produced sparks that were visible evidence of the current in the circuit and that helped generate electromagnetic waves.

Across the laboratory, Hertz had another loop attached to another \(\text{RLC}\) circuit, which could be tuned (as the dial on a radio) to the same resonant frequency as the first and could, thus, be made to receive electromagnetic waves. This loop also had a gap across which sparks were generated, giving solid evidence that electromagnetic waves had been received.

Hertz also studied the reflection, refraction, and interference patterns of the electromagnetic waves he generated, verifying their wave character. He was able to determine wavelength from the interference patterns, and knowing their frequency, he could calculate the propagation speed using the equation \(\upsilon =\mathrm{f\lambda }\) (velocity—or speed—equals frequency times wavelength). Hertz was thus able to prove that electromagnetic waves travel at the speed of light. The SI unit for frequency, the hertz (\(1 Hz=\text{1 cycle/sec}\)), is named in his honor.


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