일반물리학 2 — Diffraction: Problem 36

36 Derive this expression for the intensity pattern for a three-slit "grating": \[ I = \frac{1}{9} I_m(1 + 4 \cos \phi + 4 \cos^2 \phi), \] where \(\phi = (2\pi d \sin \theta)/\lambda\) and \(a \ll \lambda\).