That is to say when crests overlap they add to make a BIG crest, when troughs overlap they form a DEEP trough, and when a crest meets a trough, they cancel. If it is larger you do not see the interference. We observe the same result as we did with the water waves. This experiment is called the two slit diffraction experiment. The two slit diffraction experiment says that light behaves like a wave.
All electromagnetic waves travel at the same speed the speed of light.
All that remained they believed was to figure out some of the remaining details. Cathode ray tube work between 1898-1903. Study of gas discharges. Maxwell's work on electromagnetic waves has already preceded this. The particle moving between the cathode and the anode were first called cathode rays then later electrons.