Use a normalization slightly different from the text and write the complex electric field of an electromagnetic wave packet as

(a) Show that the total linear momentum of the wave packet satisfies

(b) Produce an argument which shows that

(c) When does the equal sign apply in part (b)?

E(r, t) = 1 (27)/2 fdk E dk E (k) exp[i (k. r - ckt)].