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Before Einstein, physicists thought particles and waves were two completely different phenomena. Particles appeared to be simple, but waves were clearly more complicated.
Waves are motion. They are not entities in and of themselves, but rather waves are the organized motion of a great many smaller entities. It shows in a figure a typical wave.
Waves are characterized by amplitude and wavelength. Amplitude A is how high the wave goes up above and down below its average height Wavelength w is the distance between crests. Waves move through space, and change with time. If the wave in this figure is moving to the left, its entire shape moves together as if sliding left. If we concentrate on what the wave is doing at a specific point, such as point X in the figure, we would see the wave height going up and down as time passes; it oscillates between its maximum and minimum heights. As it goes from maximum to minimum and back to maximum again, the wave completes one full cycle. If the wave goes through 9 full cycles per second, its frequency f is 9 cycles/second, or 9 Hertz, which is usually written 9 Hz. A computer that executes 5 billion instruction cycles per second has a frequency of 5 gigahertz (5GHz). The product of wavelength and frequency is always equal to the wave’s velocity: wf=v.
Before Einstein, what physicists thought they knew about particles and waves is shown in this below figure.