Answer:
Λ = 2 t
Explanation:
This is a problem is interference by reflection, where it is requested not to take into account the changes of phase and change in the wavelength within the film
λₙ = λ₀ / n
When the wave enters the film, it must reach the end of the film, be reflected and reach the initial surface, therefore the total length is
d = 2 t
where t is the thickness of the film, because the film is perpendicular to the surface
therefore the difference in optical path between the reflected ray on the surface and the end film is
Λ = 2 t