When light rays travelling from air meet  the upper surface of an oil layer, some light waves are reflected. Reflection of some light rays also occur at the lower surface of the oil layer. These two reflected rays interfer either constructively or destructively, depending on their phase difference. It should be noted that rays travelling from one medium into a more optically dense medium will have the reflected rays inverted hence destructive interference will occur. When these reflections occur from a layer of oil, for example, colourful pattern are produced.
Review the last week’s lesson on Interference
of light waves. This can be done by asking questions like:
What is interference? How is constructive interference and destructive interference caused?

 Set up an experiment for students to observe a Thin film on water. Students are to  perform the experiment, Observe, Analyze and Communicate their findings in 
  their report sheets. Rubric is given by the  teacher.

EXIT QUESTION: Use diagrams to explain why the top of a soap film appears bright from one side and dark from the other side when light is transmitted through it.

Colourful pattern on a thin film.
Source: http://en.wikipedia.org/wiki/Thin-film_interference