To my understanding, a first, second, or third order diffraction is the number of modes away from the zero order diffraction, which is no diffraction. To obtain a certain diffraction order, say the first, you would need some specific quantized energy, likewise with the second and third order...
Homework Statement
In the experiment of Davisson and Germer (a) show that the second- and third-orderdiffracted beams cannot occur and (b) find the angle at which the first-order diffracted beamwould occur if the accelerating potential were changed from 54 to 60 V. (c) What...