# Thin film question

1. May 18, 2007

### Hyacinth42

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?

2. May 18, 2007

### Staff: Mentor

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

3. May 18, 2007

### Hyacinth42

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.

4. May 18, 2007

### Staff: Mentor

Sounds reasonable.

But light is a wave!

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

5. May 18, 2007

### Hyacinth42

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

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

6. May 18, 2007

### Staff: Mentor

The "wave phenomenon" involved is interference.
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.