find the minimum thickness of a film which will strongly reflect the light of wavelength 589nm.the refractive index of the material of the film is 1.25?

Strong reflection is referring to constructive interference between rays reflected off the top and bottom of the film.

There is a phase shift of ½λ for rays reflected off the top of the film (reflection against a more dense medium)
There is no phase shift with reflection off the bottom of the film (reflection against a less dense medium)

Reflected rays off the top will combine constructively when alongside waves having a path difference of ½ wave (or 1½, 2½ ....) caused by travelling through the film.
The minimum thickness (t) for a path diff. of ½ wave is given by ..
2 t = ½ λ'' .. (λ'' = wavelength in film = λair / n)

t = ¼( λ / n) .. .. 0.25 x 589^-9m / 1.25 .. .. ?t = 1.18^-7 m

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