ask a stupid question

[Sethomas]

A slit of width 30 microns is illuminated by light of wavelength 5770 angstroms. A screen is placed 4.0 m beond the slit.
A) What is the linear width (edge to edge) of the central diffraction maximum that appears on the screen? (Note: using an equation for linear width is not a satisfactory starting point for this problem.)
B)The entire apparatus {light source, slit, screen} is immersed in a transparent liquid. This causes the 4th diffraction minimum to now appear where the 3rd minimum had appeared before the liquid was added. What is the index of refraction of the liquid?