where γ γ and γ γ ′ are the angles between n and n ′ and some direction n ″ . This condition puts a constraint on the allowed form for f ( θ θ ) , i.e., the real and imaginary part of the scattering amplitude are not independent in this case. For example, if | f ( θ θ ) | in f = | f | e 2 i α α is known (say, from the measurement of the cross section), then α α ( θ θ ) can be determined such that f ( θ θ ) is uniquely determined within the alternative f ( θ θ ) → → − − f ∗ ∗ ( θ θ ) .