# Homework Help: Thin film constructive vs destructive concept clarification

1. Apr 25, 2014

### agentlee

1. The problem statement, all variables and given/known data
So if light travels from vacuum to oil to water (1 -> 1.4 -> 1.33) how do you know if there is a constructive or destructive interference?

2. Relevant equations

3. The attempt at a solution
I know that when it goes from air to the oil there is a phase shift of 1/2. But when it goes from oil to water, there is no phase shift. How would you determine if the entire thing is constructive or destructive or not? Would you add the two phase shifts together to determine it?

2. Apr 25, 2014

### collinsmark

If the light reflects from a medium with a higher index of refraction there is 180 deg phase change.

If the light reflects from a medium with lower index of refraction there is no phase change.

There is also no phase change from the light that is simply transmitted without reflecting, and this is true whether the light is traveling into a higher index of refraction or a lower index of refraction. Transmitted rays may have path length differences, but no phase changes.

Phase changes only happen on reflections, and only when reflecting off of a medium of higher index of refraction.