Show how the wavefront curvature R(z) and the on-axis phase shift (0,z) of a Gaussian beam, written as a function of the phase retardation factor ((z) given at the end of this question, changes on going from z = 0 to large values of z. Compute R(z) and (0,z) for z = +zu/2 and z = -Zo Gaussian beam formulae for Q2. Beam waist: W, - (4), Beam radius: W (z) = W. [1 + ( Radius of curvature: R(z) = z[1+ ()]. Phase retardation: