Question on Thin Film Interference

In summary, the problem is that the student is missing the step of solving for t using lambda x 0.25 in order to get the thickness of the film.
  • #1
tigerguy
32
0
Hi - I'm having trouble with the following problem:

A mixture of yellow light (wavelength = 574 nm in vacuum) and violet light (wavelength = 418 nm in vacuum) falls perpendicularly on a film of gasoline that is floating on a puddle of water. For both wavelengths, the refractive index of gasoline is n = 1.40 and that of water is n = 1.33. What is the minimum nonzero thickness of the film in a spot that looks the following colors because of destructive interference, for yellow light?

Because its destructive interference, I'm using the equation t = lambda/2 ,but this is leading to the wrong answer. I'm also making the wavelength for the yellow light equal 410 (574/1.40). Where am I going wrong, or am I even using the right equation? Thanks for your help!
 
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  • #2
Do you know about any change in phase of a wave due to reflection.

M.P.
 
  • #3
I think there's a change between the air and oil, but no change between the oil and water? Is that right?
 
  • #4
yes the chang in phase by Pi if the wave is reflected from a denser medium.
 
  • #5
I still don't understand where I'm going wrong. Theres only one phase change, so the value of lambda will change. Then, I divide that value by 2, and I should get my thickness. But, the answer is wrong - why?

I keep on getting 205 nm for the yellow wavelength.
 
  • #6
Your method and answer seem OK to me. Why do you think it's wrong?
 
  • #7
It's an online assignment, and it was marked incorrect - which is why I'm confused.
 
  • #8
Should I maybe try t = lambda x 0.25?
Or, perhaps, my lambda value is incorrect. I'm dividing by 1.40 (to reprsent the gasoline). Is it possible that I should divide by 1.33 first?

I'm feeling really lost on this one, I don't understand why Webassign keeps marking it wrong.
 
  • #9
tigerguy said:
Should I maybe try t = lambda x 0.25?
If you can afford multiple attempts, I would try that for fun. But that answer is wrong. (But it wouldn't surprise me that they had it backwards.)

Or, perhaps, my lambda value is incorrect. I'm dividing by 1.40 (to reprsent the gasoline). Is it possible that I should divide by 1.33 first?
Nope; that would make no sense.

I'm feeling really lost on this one, I don't understand why Webassign keeps marking it wrong.
It can be picky. Exact format and units might matter. And it's been plain WRONG before.

What was the purpose of giving the information about the violet light? It doesn't enter into the problem. Did you provide the complete statement of the question?
 
  • #10
Hmm - the exact question is:

What is the minimum nonzero thickness of the film in a spot that looks the following colors because of destructive interference?

a. Yellow
b. Violet

I didn't enter it first because I figured the steps for solving both would be the same. Thanks for your help, again.
 
  • #11
Ah... that's much different! If it looks yellow, that means it's the violet light that's destructively interfered! And vice versa.
 
  • #12
Ohhhh I understand now! Thanks!
 

1. What is thin film interference?

Thin film interference is a phenomenon that occurs when light waves interact with a thin layer of material, causing a change in the color or intensity of the light. This is due to the constructive and destructive interference of the light waves as they reflect and refract through the film.

2. What factors affect thin film interference?

The thickness of the film, the refractive index of the film and the surrounding medium, and the angle of incidence of the light are all factors that affect thin film interference. Additionally, the wavelength of the light and the polarization of the light can also play a role.

3. How is thin film interference used in practical applications?

Thin film interference is used in various industries, including electronics, optics, and coatings. It is used to create anti-reflective coatings on eyeglasses and camera lenses, as well as to produce colorful patterns on soap bubbles and oil slicks. It is also utilized in the production of thin film solar cells.

4. Can thin film interference be observed in everyday life?

Yes, thin film interference can be observed in everyday life. For example, when you see a rainbow pattern on a soap bubble or a colorful sheen on a puddle of oil, you are witnessing thin film interference in action.

5. How is thin film interference different from other types of interference?

Thin film interference is distinct from other types of interference, such as diffraction or interference from multiple slits, because it specifically involves the interaction of light waves with a thin film. The interference patterns produced by thin film interference are unique and depend on the properties of the film, while other forms of interference may produce different patterns.

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