A layer of oil, with a refractive index of 1.40798, 0.91820 mm thick, floats on water. (a) Light of vacuum wavelength 632.800 nm, emitted by a stabilized He-Ne laser, is reflected at normal incidence by the oil. How many wavelengths are contained in the light wave passing back and forth through the oil?

A layer of oil, with a refractive index of 1.40798, 0.91820 mm thick, floats on water. (a) Light of vacuum wavelength 632.800 nm, emitted by a stabilized He-Ne laser, is reflected at normal incidence by the oil. How many wavelengths are contained in the light wave passing back and forth through the oil?