North Carolina RADAR State Practice Exam

The knowledge that radio waves could reflect off solid objects dates back to the work of Heinrich Hertz in the late 1880s. Specifically, around 1886, Hertz conducted experiments that demonstrated the existence of electromagnetic waves, which included radio waves. Through his experiments, he was able to show that these waves could indeed reflect off surfaces, similar to how light reflects. This foundational work laid the groundwork for future developments in radio and radar technologies.

The choices indicating later years pertain to advancements and applications of radio wave technology that occurred after Hertz's initial discoveries. Therefore, identifying 1886 as the year when it was first known that radio waves could reflect from solid objects is rooted in the historical context of Hertz's experiments, making it the correct choice.