# Thin film question

I need help with this question:

"Name the wave phenomenon that is responsible for the formation of regions of different color when white light is reflected from a thin film floating on water."

In my handy-dandy high-school physics book, it says that "The different thickness of the gasoline is what causes the different regions of different color."

This is not a phenomenon of light, but rather a phenomenon of the gasoline.... I know how thin films work, how some bounces off, experiences a phase change and then interferes deconstructively with light bouncing off the water while in the oil, but this does not really explain the different regions of different colors... So how do I accurately answer the question?

Doc Al
Mentor
Hints: How does the thickness of the film affect the interference? Are all colors (wavelengths) equally affected?

No, of course not, because of the different wavelengths and diferent n values some deconstruct and others don't. So then, a suitable answer would be something like:
Different wavelengths of light bounce off differently, and depending on the thickness of the gasoline, different colors deconstruct and thus regions of different color are formed?

That doesn't sound right, it's a property of white light, not a property of a wave... Oh! OH! Could it be the fixed end thing? Because a wave experiences a phase change when it hits a fixed end (or when light goes from a smaller n to a larger n) that is what causes the light to deconstruct on a thin film.

Doc Al
Mentor
No, of course not, because of the different wavelengths and diferent n values some deconstruct and others don't. So then, a suitable answer would be something like:
Different wavelengths of light bounce off differently, and depending on the thickness of the gasoline, different colors deconstruct and thus regions of different color are formed?
Sounds reasonable.

That doesn't sound right, it's a property of white light, not a property of a wave...
But light is a wave!

Oh! OH! Could it be the fixed end thing? Because a wave experiences a phase change when it hits a fixed end (or when light goes from a smaller n to a larger n) that is what causes the light to deconstruct on a thin film.
But the phase changes upon reflection are the same no matter what the thickness.

But light is a wave!

Yes, light is a wave, but they ask "The wave phenomenon", I read that as general wave phenomenon rather than light wave phenomenon.

But the phase changes upon reflection are the same no matter what the thickness.

Yes, and thus, it is a wave phenomenon rather than a light wave and thin film phenomenon.

Doc Al
Mentor
Yes, light is a wave, but they ask "The wave phenomenon", I read that as general wave phenomenon rather than light wave phenomenon.
The "wave phenomenon" involved is interference.
Yes, and thus, it is a wave phenomenon rather than a light wave and thin film phenomenon.
The phase change upon reflection is certainly a "wave phenonmenon"; but since it's the same for all thickness it, by itself, cannot explain the different colors.