What does a "phase reversal" really do?

[cmlee1324]

While learning about thin film interference, I see diagrams quite often that display waves being inverted. For instance, a wave that bounces off outside the film is inverted because it is going from air to film (lower to higher index of refraction) while a wave that bounces INSIDE the film is not inverted (higher to lower index of refraction). This got me thinking.

Why do phase "reversals" (what my teacher calls a phase change of 180 I think) matter? If light is still in the same location at the same angle, why does the phase matter? Does light appear different when it has a different phase? If the period of light is so ridiculously fast, why would a nanosecond difference in phase appear different?

If this question makes no sense from the view of someone with more knowledge on the subject, may I request a very basic explanation of waves and how they interact with thin films and an eye?

 

[tech99]

A phase shift of 180 degrees is not visible, but if we have two superimposed rays of light having opposite phase then we will find cancellation.

 

[cmlee1324]

So the only purpose of taking phase into account is to see if there is any destructive interference?

If so, why would that be necessary when talking about thin film interference? I would assume that there are no interactions between the rays; they glance off into infinity.

Do the rays interact? Is that the reason why 1 vs 2 phase reversals gives different equations for constructive and destructive interference? (m or m+1/2)

 

[DaveE]

So the only purpose of taking phase into account is to see if there is any destructive interference?

Yes, pretty much. At least in a general interpretation. Although it's "interference" not just destructive interference.

If so, why would that be necessary when talking about thin film interference? I would assume that there are no interactions between the rays; they glance off into infinity.

Some may go different places. But some paths meet at a cell on your retina, or the photocell in your instrument. At that point the response they create is the result of addition, in which case phase can be really important. Like adding two vectors up, direction matters. Same with light.