Thin Films:

  

Figure 21: Reflection by Thin Film

For n > n₁ and n > n₂:
destructrive interference occurs when      $2t = m\displaystyle\frac{\lambda}{n}$
constructive interference occurs when     $2t = (m+1/2)\displaystyle\frac{\lambda }{n}$
NOTE: $180^\circ$ phase change on reflection of more dense material