(1) Sketch out the standard representation for thin-film interference: two horizontal lines, representing 3 separate media. Label on this sketch the indices of refraction of the three media. The middle medium is the "thin film."

(2) Determine the wavelength $\lambda_2$ of your wave inside of the thin film. Remember that maximum constructive interference has to do with whether or not some integer-number of wavelengths fit into some distance, so it is important to know what the wavelength is inside of the medium in which the path-length-difference occurs - which, if you use the standard sketch, is always the middle (or second) medium.

(3a) Determine which equation is appropriate for describing constructive interference. To do this, determine whether or not there is a relative phase-shift between the two waves that are interfering. Remember that, at every boundary, some light is transmitted while some is reflected. The reflected light will have a $\pi$ phase-shift if the index of refraction of the medium the wave is traveling into is greater than that which it is leaving. Using the standard sketch, this means the wave reflected off the top surface will have a phase-shift iff $n_2 > n_1$ and the wave reflected off the second surface will have a phase-shift iff $n_3 >n_2$. Transmitted light does not undergo a phase-shift.

(3b) If there is a $\pi$ relative phase shift, then: 

• constructive interference is described by $2t = \left(m + \frac{1}{2}\right) \lambda $, where $t$ is the thickness of the thin film and $\lambda$ is the wavelength of the light within the thin film (so here it takes the value of $\lambda_2$ calculated above, but I have left it as $\lambda$ to be consistent with other resources). 
• destructive interference is described by $2t = m \lambda$

If there is no relative phase shift between the two waves, then the equations are swapped and $2t = m \lambda$ corresponds to constructive and $2t = \left( m + \frac{1}{2} \right) \lambda$ corresponds to destructive interference.

You now have all of the variables you need to finish the problem. Depending on the problem, you may be asked for a couple of different things. One might be a "minimum thickness" - in this case, you want to find the minimum possible value you can find for $t$, which corresponds to the smallest possible $m$ value (usually either $0$ or $1$). Or, you may be asked what wavelengths would produce destructive or constructive interference given some thickness - which is as simple as solving $2t = m \lambda$ for $t$ (or the other equation, depending on the situation). Once you have gone through step 3 above, you have all of the info that can be gleaned algorithmically: the rest is down to correctly interpreting the individual problem and what it is asking for. 

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