See attached for exact description and figure.
In the illustration above, two glass plates have a very slight angle between them because a speck of dust props up one edge of the top plate. When illuminated from above with light of wavelength 546 nm, eleven dark interference fringes are observed (counting from zero at the left edge).
a. Find the height of the dust speck.
b. Find the angle (in degrees) between the plates if their diameters are 2.00 cm.
Please see the solution to your problem in the attached word file ...
It shows how to find the height of a dust speck between two glass plates by inteference. The solution is detailed and was rated '5/5' by the student who posted the question originally.