Hello,

When light reaches a boundary between two media, some of the light will pass through the boundary while some portion of it will reflect. If the boundary is a thin film (e.g. thin layer of oil sitting on top of a water puddle), it is possible that the light waves that passed through (i.e. transmitted into the new medium) and the light waves that reflected could be in-phase and add up, this is known as constructive interference. Where the film is particularly thin, it is possible that the transmitted and reflected light waves are out-of-phase, and this would result in destructive interference. If you have any further questions, please email the instructor and somebody will get back to you.