A layer of oil, with a refractive index of 1.40798, exactly 1 mm thick, floats o

Question

A layer of oil, with a refractive index of 1.40798, exactly
1 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
con tained in the light wave passing back and forth
through the oil?
(b) Does the light reflected from the two surfaces of
the oil interfere constructively or destructively?

The answer to A is: 4450 wavelengths

The answer to B is: Destructive

What I don't know is why. Please explain why BOTH of these answers are true to get credit.

Explanation / Answer

1) number of wavelengths = distance traveled/wavelength in material = 2d/(lambda/n) = 2*1.0E-3*1.40798/632.8E-9= 4450 wavelengths

since there is one low n to high n reflection, the equation for destructive is m lambda = 2 d

so destructive

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