I understand the derivation and calculation of de Broglie wavelengths. What I don't understand is what exactly they are. What does the wavelength of a particle mean? Does it mean the wavelength of its probability wave, or some other kind of wave? The idea of wave-particle duality for EMR makes sense to me in that it can be seen as oscillating changes in energy (a wave), and as photons from the photoelectric effect. But with particles, what quantity is oscillating in their wave? Thanks!