The wavelength of a probe is roughly a measure of the size of a structure that it can probe in some detail. The quark structure structure of protons and neutrons appears at the minute length scale of `10^-15m` or less. This structure was probed in early 1970's using high energy electron beams produced by a linear accelerator at stanford, USA. Guess what might have been the order of energy of these electrons beams. (Rest mass energy electron=0.511 MeV).