I have learnt that the de Broglie's as *λ*=*h*/*p* (where *h* is Planck's constant,*p* is the momentum of the particle), we can derive it from equating Einstein's mass-energy equivalence and the energy of a photon, *E*=*h**ν, then replacing 'c' with the velocity of any particle*.

Waves are associated with moving particle, so as the wave length. The equation $E=mc^2$ is applicable for particles at rest; I recently found that the general eqn is $E^2=(mc^2)^2+(pc)^2$ Why do we use $E=mc^2$, when $E^2=(mc^2)^2+(pc)^2$is the eqn for moving particles? A particle at rest cannot have a wave length - isn't that so?