Fraunhofer approximation
= Fraunhofer approximation
{c}
{title2=$D\gg a^2/\lambda$}
The far-field approximation neglects quadratic variation of propagation phase across an aperture of characteristic size a at observation distance D. For two narrow slits separated by s, their path difference remains $s\sin\theta$ at a finite observation angle. The additional substitution $\sin\theta\simeq Y/D$ requires a small observation angle; it does not follow from the <Fraunhofer approximation> itself.