The students will use their crystal radios to “catch” electromagnetic radiation (EMR) at about 1,000,000 Hz, the AM radio band. There are two types of EMR, natural and man made. Marconi made the first radio station in 1895, before that there was only natural EMR. Naturally, stars are constantly emitting EMR across the entire spectrum, although the majority is in the form of infrared, visible light, and ultraviolet. We can only see visible light, all the rest remains invisible to us. However, the crystal radio can detect man made radio waves from a local station. It is a thrill to make this EMR and detect invisible EMR.
see https://www.youtube.com/watch?v=lwfJPc-rSXw