An oil film (n_f = 1.45) of thickness t floating on water (n=1.33) is illuminate

Question

An oil film (n_f = 1.45) of thickness t floating on water (n=1.33) is illuminated by white light at normal incidence. Determine the path difference for the light reflected from the top of the film with that from the bottom of the film in terms of thickness t, n_f and the wavelength of the light in vacuum (reflectance) Determine the minimum thickness of the film for the red light (lambda=600nm)most strongly reflected. A beam of light of wavelength 600nm passes through two closely spaced glass plates. The transmitted light is bright for what minimum plate separation d?

Explanation / Answer

a) path difference = 2 * nf * t

b)    Here,   2 * nf * t = (2m + 1) * lambda/2

=> 2 * 1.45 * t = 600/2

=> minimum thickness of film , t = 103.448 nm

c)   minimum plate separation = (600/2)

                                                = 300 nm

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