I tried to do it with phi(path)+phi(refl)=piBut it does not work. Would you be able send me a explained solution thank you

A broad beam of light of wavelength 542 nm is sent directly downward through the glass plate (n = 1.47) in Figure (a). That plate and a plastic plate (n = 1.26) form a thin wedge of air that acts as a thin film. An observer looking down through the top plate sees the fringe pattern shown in Figure (b), with dark fringes centered on the ends. (a) What is the thickness (in m) of wedge at the right end? (b) How many dark fringes will the observer see if the air between the plates is replaced with a liquid with n = 1.43? Incident light (a) (b) (a) Number 1626E-9 Units m (b) Number 8 Units No units +