Thin film interference problem

In summary, a high school physics student is struggling with thin film interference and is given a problem involving a thin layer of oil floating on water. By using the formula noil/nwater = λwater/λoil, the student finds that the thickness of the oil in the region that strongly reflects green light (λ=556nm) is 296 nm. However, the correct answer is 222 nm, which can be achieved by using n air instead of n water in the formula.
  • #1
Terraist
9
0
Hi, I'm a high school physics student and I'm having a lot of trouble with thin film interference

Homework Statement



A thin layer of oil (noil=1.25) is floating on water (n=1.33). How thick is the oil in the region that strongly reflects green light (λ=556nm)?

Homework Equations



noil/nwater = λwater/λoil

t = mλ/2, where t is thickness of the film, m is order of interference maxima (i'm not sure about this one)

The Attempt at a Solution



1.25/1.33 = 556/λ
λ=592 nm

t=mλ/2
=(1)(592)/2
=296 nm

(ans in book 222 nm)

I'm not entirely sure of what I'm doing here. I'm trying to just find the right formula to plug the variables in and I don't grasp the principles behind it. If someone could shed some light on the subject and provide some tips on how to solve similar problems it would be greatly appreciated.
 
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  • #2
Everything is right except you're not supposed to use n oil and water but rather n air and oil, because the ray is coming from above in air and not from the water right?( I think this is a trick question or something) So your new wavelength would be 556/1.33 (n air is 1) = 444 and you should get the books answer.
 

1. What is thin film interference?

Thin film interference is a phenomenon that occurs when light waves reflect off of the top and bottom surfaces of a thin film and interfere with each other. This results in changes in the intensity and color of the reflected light.

2. What causes thin film interference?

Thin film interference is caused by the difference in the refractive index between the two materials that make up the thin film. This difference in index causes variations in the speed of light as it passes through the film, resulting in the interference of the light waves.

3. How does thin film thickness affect interference?

The thickness of the thin film plays a crucial role in thin film interference. When the thickness of the film is a multiple of the wavelength of the incident light, constructive interference occurs, resulting in bright colors. On the other hand, when the thickness is an odd multiple of half the wavelength, destructive interference takes place, leading to dark colors.

4. What is the difference between thin film interference and Newton's rings?

Thin film interference and Newton's rings are both examples of interference phenomena, but they occur due to different reasons. Thin film interference happens when light waves reflect off thin films, while Newton's rings occur when light reflects off a curved surface, such as a lens or a convex mirror.

5. How is thin film interference used in real-life applications?

Thin film interference has various practical applications, including anti-reflective coatings on glasses and camera lenses, optical filters, and thin film solar cells. It is also used in non-destructive testing techniques, such as ellipsometry, to measure the thickness of thin films in industrial settings.

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