# Index of refraction Thin Films

1. Mar 21, 2014

### blue_lilly

1. The problem statement, all variables and given/known data
The diagram shows light incident from above on a film of thickness d. Each of the three materials in the figure can be chosen to be air, with index of refraction n=1.00, water, with index n=1.33, or glass, with index n=1.50. Under which of the following conditions will the reflected light be completely or nearly eliminated by interference?
(lambda2 refers to the wavelength of the light inside the thin film.)
(Note: In the case that d<<lambda2, you can assume the thickness is so small that the travel distance in material 2 has negligible affect on the phase of the wave. Treat this as if the two interfaces are almost right on top of each other.)

1) T/F d=lambda2/4, material 1 is air, 2 is water, 3 is glass.
2) T/F d=lambda2/2, material 1 is air, 2 is water, 3 is glass.
3) T/F d=lambda2/4, material 1 is glass, 2 is air, 3 is glass.
4) T/F d=lambda2/4, material 1 is water, 2 is glass, 3 is air.
5) T/F d<<lambda2, material 1 is water, 2 is glass, 3 is air.

2. Relevant equations

2*n*d*sinθ = (m-(1/2))*λ
m=integer
λ=wavelength
n=index of refraction = speed of light in vacuum/speed of light in medium
d= thickness

3. The attempt at a solution
We are looking for places where the refracted light is eliminated by interference, so we are looking for destructive interference.
Destructive if Path-length Difference(PLD) 2nd= (m + (1/2))λ when the sources are exactly in phase.
Destructive if Path-length Difference(PLD) 2nd = mλ when the sources are exactly out of phase.
There are phase reversals when it is reflecting of a higher n.

1) d=lambda2/4, material 1 is air(n=1.00), 2 is water, 3 is glass.
True because
There would be 2 phase reversals [1 between air and water and the other between water and glass] So the wavelength would be exactly in phase which means it needs to have PLD of (1/2)λ
d=2λ/4
(2λ/4) = (1/2)λ
(2/4)λ= (1/2)λ
(1/2)λ= (1/2)λ

2) d=lambda2/2, material 1 is air(n=1.00), 2 is water(n=1.33), 3 is glass(n=1.50).
False because
There would be 2 phase reversals [1 between air and water and the other between water and glass] So the wavelength would be exactly in phase which means it needs to have PLD of (1/2)λ
d=2λ/2
2λ/2 = (1/2)λ
(2/2)λ = (1/2)λ​

3) d=lambda2/4, material 1 is glass(n=1.50), 2 is air(n=1.00), 3 is glass(n=1.50).
True because
There would be 2 phase reversals [1 between air and glass and the other between air and glass] So the wavelength would be exactly in phase which means it needs to have PLD of (1/2)λ.
d=2λ/4
(2λ/4) = (1/2)λ
(2/4)λ= (1/2)λ
(1/2)λ= (1/2)λ

4) d=lambda2/4, material 1 is water(n=1.33), 2 is glass(n=1.50), 3 is air(n=1.00).
True because
There would be 2 phase reversals [1 between air and water and the other between the water and glass] So the wavelength would be exactly in phase which means it needs to have PLD of (1/2)λ.
d=2λ/4
(2λ/4) = (1/2)λ
(2/4)λ= (1/2)λ
(1/2)λ= (1/2)λ

5) d<<lambda2, material 1 is water(n=1.33), 2 is glass(n=1.50), 3 is air(n=1.00).
False because
There would be 2 phase reversals [1 between air and water and the other between the water and glass] So the wavelength would be exactly in phase which means it needs to have PLD of (1/2)λ.
d<<lambda2
λ2= (1/2)λ

I thought i was doing right but the answer is incorrect.
Any help would be greatly appreciated.

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2. Mar 21, 2014

### TSny

Hello.

You are not getting the number of phase reversals correct for some of the cases. How do you decide if a phase reversal occurs?

3. Mar 21, 2014

### blue_lilly

A phase reversal happens when the n it is "hitting" is larger then what the "n" was before.

1) d=lambda2/4, material 1 is air(n=1.00), 2 is water(n=1.3), 3 is glass(n=1.5).
So for this one the wave is traveling in air and then it hits water, which has a higher "n" value. this means that part of the wave is reflected back and part continues through the water. The wave traveling in the water "hits" the glass and glass has a higher "n" value then water so part of the wave reflects back and some continues through the object.

I thought this was how you are supposed to tell. Is it incorrect?

4. Mar 21, 2014

### TSny

Yes. You got this one right. But some of the other cases are incorrect. For example, check case(3).