4. The double slit shown in figure Q4 is illuminated by a laser with a wavelength of 500 nm. (a) Determine the mathematical form of the Fraunhofer diffraction pattern. (b)

How many bright fringes will be displayed within the central lobe? (c) Sketch the diffraction pattern. (d) A thin transparent plate is introduced immediately in front of one of the slits only. It results in an optical phase difference of t radians. What happens to the fringe pattern?