Interference

[MedPR]

This is one of the trickiest simple questions I've seen in TBR.

Soap bubbles change colors when observed over time. This is MOST likely due to:

A. Refraction
B. Evaporation
C. Dispersion
D. Selective Absorption

Go! Answer Here: B

Explanation to come. Explanation is in white for now, since not too many people have had a chance to try it without seeing the answer/explanation.

Constructive interference is based on a path difference traveled by the two waves (the wave that reflects off the front of the soap film and the save that reflects off of the back of the soap film). As the soap film evaporates over time, the thickness of the soap film changes (perhaps getting thicker or thinner). This change in soap film thickness in turn changes the path difference for the two waves, so as the soap film evaporates, different colors will be the ones that are adding constructively. Here's where it gets interesting Choices A and C are incorrect, because choice C is saying essentially the same thing as choice A, a refractive effect -- it occurs when the index of refraction for a material varies with the wavelength of light. When two answer choices are fundamentally the same from a physical point of view, those two choices are usually both wrong, and thus should be eliminated. Choice D is incorrect, because the wavelengths of light that are selectively absorbed by a material depend upon the molecular makeup of the material. Since this makeup does not change much with time, the colors absorbed should not change much with time.

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[milski]

I don't like it. Water does not evaporate that fast. The thinning of the layer happens mostly because the liquid flows down to the bottom of the bubble.

 

[gettheleadout]

Without looking your answer spoiler, I would eliminate A and D because they don't to be time-sensitive. Between B and C, I can't think of a reason why evaporation would change the color, but I recall seeing fluid movement on bubble surfaces when you watch them float, so maybe dispersion? C?

Edit: lol, oh well. I was using "dispersion" rather loosely anyway haha.

 

[MedPR]

I don't like it. Water does not evaporate that fast. The thinning of the layer happens mostly because the liquid flows down to the bottom of the bubble.

Without looking your answer spoiler, I would eliminate A and D because they don't to be time-sensitive. Between B and C, I can't think of a reason why evaporation would change the color, but I recall seeing fluid movement on bubble surfaces when you watch them float, so maybe dispersion? C?

Edit: lol, oh well. I was using "dispersion" rather loosely anyway haha.

I picked C as well. One sec I'll put the explanation in the OP. It's rather genius, but equally lame.

 

[milski]

The "official" dispersion is the fact that different wavelengths of light have different refracting index for the same media. That one does not change with time either.

 

[MedPR]

The "official" dispersion is the fact that different wavelengths of light have different refracting index for the same media. That one does not change with time either.

I figured dispersion was what could account for the fact that if the bubble deforms for any reason the colors would be different. Obviously not using the same definition of dispersion that you are using, but I think I've read somewhere that variations in shape of the same media can affect the wavelength of the light.

 

[gettheleadout]

The "official" dispersion is the fact that different wavelengths of light have different refracting index for the same media. That one does not change with time either.

Haha, my current lack of any E&M physics knowledge is quite apparent at times...

 

[milski]

I figured dispersion was what could account for the fact that if the bubble deforms for any reason the colors would be different.

It's all about the thickness of the bubble wall and which wavelengths have constructive interference for that thickness.

Evaporation is a nice try but does not explain the patterns that can be observed in a vertical flat bubble - bands of colors which get narrower and closer to the bottom with time. It's still the only time-dependent choice. I guess sometimes the best answer is not the most correct but the least problematic one.

 

[chiddler]

isn't dispersion in a prism like system?

er...what is dispersion?

I guessed correctly because I recall that the thickness of the thin layer changes the frequency of the light that is reflected. evaporation can change thickness so frequency can change.

 

[MedPR]

isn't dispersion in a prism like system?

er...what is dispersion?

I guessed correctly because I recall that the thickness of the thin layer changes the frequency of the light that is reflected. evaporation can change thickness so frequency can change.

Yea, it's the prism stuff. I guess it's governed by nlambda=nlambda..?

 

[milski]

isn't dispersion in a prism like system?

er...what is dispersion?

I guessed correctly because I recall that the thickness of the thin layer changes the frequency of the light that is reflected. evaporation can change thickness so frequency can change.

The rainbow that you can get with a prism and white light going in it is an example of dispersion. Each wavelength has slightly different refraction index and as a result each color exits the prism at a slightly different angel, effectively separating them.

The colors are actually interference based. Some light reflects from the top of the bubble layer, some goes through it and reflects from the bottom. Based on the thickness of the layer, only a certain wavelength will have a constructive interference between the two reflections, giving you the color patters.

 

[chiddler]

Yea, it's the prism stuff. I guess it's governed by nlambda=nlambda..?

from old notes:

2t = m(lambda final) for constructive
2t = m(lambda final + 1/2) for destructive

t = thickness of film

wow i just read the explanation. that's some fine test-taking logic.

 

[MedPR]

from old notes:

2t = m(lambda final) for constructive
2t = m(lambda final + 1/2) for destructive

t = thickness of film

wow i just read the explanation. that's some fine test-taking logic.

Yea it is! Sometimes there is a really fine line between identical answers and very similar answers and usually when two answers are very similar one of them is the answer.. so it's hard to commit to the logic they used.

 

[milski]

What's their logic? I know retyping sucks, just a summary would be interesting too.

 

[MedPR]

What's their logic? I know retyping sucks, just a summary would be interesting too.

It's in white in the OP so people can attempt to answer without seeing the answer.

 

[MrNeuro]

LOOOOOOL i remember this question i never lose my cool but i thoroughly remember reading this answer and just losing my cool.

I remember going through the answer choices going NO way is it evaporation b/c usually when i think of bubbles its like 3 seconds then they pop...and no i dont see color changes TBR ....

i still think dispersion is the best choice because its the phenomenon of dispersion that yields the different colors... but whatever TBR just has some twisted answers sometimes. Glad to see i wasn't the only one who had a WTF moment with this question.

 

[milski]

LOOOOOOL i remember this question i never lose my cool but i thoroughly remember reading this answer and just losing my cool.

I remember going through the answer choices going NO way is it evaporation b/c usually when i think of bubbles its like 3 seconds then they pop...and no i dont see color changes TBR ....

i still think dispersion is the best choice because its the phenomenon of dispersion that yields the different colors... but whatever TBR just has some twisted answers sometimes. Glad to see i wasn't the only one who had a WTF moment with this question.

Dispersion would work for a single ray of light. You can see the color under regular diffuse light which makes it not a very plausible explanation.

The major thickness change occurs because the fluid tends to flow down and collect at the bottom of the bubble. That's actually why they pop after a few seconds - the top becomes too thin to hold it together. There's probably some evaporation happening so while not an ideal explanation, TBR is correct in some sense.

 

[411309]

I put C too and also got it wrong. I had no idea at all it was evaporation.

 

[MrNeuro]

Dispersion would work for a single ray of light. You can see the color under regular diffuse light which makes it not a very plausible explanation.

The major thickness change occurs because the fluid tends to flow down and collect at the bottom of the bubble. That's actually why they pop after a few seconds - the top becomes too thin to hold it together. There's probably some evaporation happening so while not an ideal explanation, TBR is correct in some sense.

Isn't dispersion just the change in velocity of a wavelength of light due to its frequency (which is proportional to its wavelength) if so couldn't you argue that the reason you see color changes is because of different wavelengths of light having constructive and destructive interference due to the underlying phenomenon of dispersion?

 

[milski]

Isn't dispersion just the change in velocity of a wavelength of light due to its frequency (which is proportional to its wavelength) if so couldn't you argue that the reason you see color changes is because of different wavelengths of light having constructive and destructive interference due to the underlying phenomenon of dispersion?

That's very possible for two rays of light but the color will be virtually random. When you combine all the wavelengths and directions from which the light is coming you'll get a mix which you'll observe as white light.

The interference of reflected rays works better because the wavelength for which constructive interference happens depends only on the thickness of the bubble wall. That allows you to have the same wavelength for relatively large areas of the bubble which give you the opportunity to observe its effect.

 

[MrNeuro]

That's very possible for two rays of light but the color will be virtually random. When you combine all the wavelengths and directions from which the light is coming you'll get a mix which you'll observe as white light.

The interference of reflected rays works better because the wavelength for which constructive interference happens depends only on the thickness of the bubble wall. That allows you to have the same wavelength for relatively large areas of the bubble which give you the opportunity to observe its effect.

ahhhhh totally overlooked the whole different directions thing i knew thickness had to play a role but evaporation just didn't do it for me at the time

 

[sshah92]

ahhhhh totally overlooked the whole different directions thing i knew thickness had to play a role but evaporation just didn't do it for me at the time

I know this is a year old but... I got the wrong answer too... made me want to yell at my paper. Aaaaah