A(r, t) = (ax + iby)A 0 () exp(i) where = k(z + ct) is a

A(r, t) = (ax̂ + ibŷ)A₀(ζ) exp(iζ) where ζ = k(z + ct) is a vector potential wave packet.

(a) Find the (real) electric and magnetic fields in the approximation that the envelope function A(ζ) varies slowly over one wavelength of the wave.

(b) Find the linear momentum density carried by the wave in part (a). What does the overall algebraic sign of this quantity tell you?