Final answer:
The earliest radio broadcasts used AM radio waves, with broadcasting technology that came into widespread use in the 1920s. AM radio, which was the dominant form before the popularity of FM radio after WWII, used amplitude modulation to convey information.
Step-by-step explanation:
The earliest radio broadcasts used AM radio waves, a form of electromagnetic radiation that was first developed for public broadcasting. AM stands for amplitude modulation, where the information is conveyed by varying the amplitude of the radio waves. This type of broadcasting began in the 1920s when radio technology started to have a significant impact on society through news dissemination and commercial radio shows. The wavelengths of AM radio can span a large range, generally from one to hundreds of meters. Although later FM radio, which stands for frequency modulation, provided clearer sound with less static due to its wider signal bandwidth, the foundations of radio technology and broadcasting relied on AM wave transmissions.
Before the widespread adaptation of FM broadcasting after WWII, AM radio was the dominant mode for carrying audio communications. The technology for AM radio wave broadcasting was the result of pioneering work in the late 1800s and its cultural impact peaked with popular radio shows supported by advertising in the mid-1920s.