Dictionary:Obliquity factor

(∂ blik’ wi tē) A factor derived by Stokes for use with Huygens’ principle to show that the major energy in a wave moves forward and to prevent backward propagation:

$cos{\frac {\phi }{2}}\approx {\frac {1+cos{\phi }}{2}}$,

where $\phi$ is the angle between the directions of reradiation and of approach.