A piece of paper is placed at the end of an air wedge 4 cm long. Interference fringes appear when the light of wavelength 639 nm is reflected from the wedge. A dark fringe occurs at both the vertex of the wedge and at its paper end, and 56 bright fringes appear between. Calculate the thickness of the paper.
delta X = L (lambda / 2t)
where t is the thickness of the paper
lambda = wavelength
l = length of air wedge
X = distance between fringes.