If light is a particle, what is "wavelength"/"frequency"? Hello, Is the wavelength of a particle simply the wavelength of the psi-function before measurement? Or (/and?) is it an intrinsic property that is "even there in the particle" when you measure the presence of the particle (i.e. reduction), like energy? Or is the nature of this question exhibiting a deeper misunderstanding of mine? My formal knowledge about QM doesn't extend beyond an introductory course.