Young's Double Slit

Let us begin by looking at a thin layer of oil floating on top of water.

Light is reflected from the first surface of a thin film (at point A in this diagram). And some of the light is refracted and passes through the film and continues on to the second surface where it is again reflected (at point B in this diagram). This light passes through the film again and some of it is refracted (at point C in this diagram) and continues on with and interferes with the light originally reflected at the first surface.

The angle if incidence does make a difference.

However, we will restrict our present attention to the situation shown here, where the light is nearly perpendicular to the thin film.

With air on both sides of this thin soap film, the characters of the two reflections are different. One is from a material of low index of refraction to a material of high index of refraction (n₁ < n₂) while the other is from a material of high index of refraction to a material of low index of refraction (n₂ > n₃). These are like the reflections of a pulse in a rope with a fixed end or with a free end. This means there is an extra "phase shift" of one-half a wavelength. This changes our conditions for maxima and minima.