Questions from competitive examinations (wave optics)

Questions from competitive exams—solved in my class on 18/9/09 The difference in number of wavelengths, when yellow light ( λ= 6000 Ă) propagates through air and vacuum column of the same thickness is 1. If the R.I. is 1.003 , what is the thickness of the air column ? 2)Two coherent beams of wavelength 5000 Ă reaching a point would individually produce intensities 1.44 and 4 units. If they reach there together , the intensity is 10.24 units. Calculate the lowest phase difference with which the beams reach the point. 3) What if the beams are incoherent?