# Thin film interference

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1. Dec 3, 2014

### meaowwuff

1. The problem statement, all variables and given/known data
A coating of n1 = 1.27 material is to be added to n2 = 1.50 glass, in order to make it "irridescent."
a) What is the thinnest layer of material which will have a reflection maximum for normal-incidence light at BOTH 400 and 600 nm (vacuum) wavelength?
b) This thickness will have a reflection minimum at some wavelength between 400 and 600 nm. What is the wavelength of the reflection minimum?

2. Relevant equations

2*d=lambda(coating), 2*d=lambda(coating)/2

3. The attempt at a solution

I got the first part correct and the answer was 472 nm but I can't get b right. For b I did 2*d=lambda(coating)/2 since it says that the reflection will be minimum.

2. Dec 3, 2014

### ehild

2. Relevant equations

2*d=lambda(coating), 2*d=lambda(coating)/2

3. The attempt at a solution

I got the first part correct and the answer was 472 nm but I can't get b right. For b I did 2*d=lambda(coating)/2 since it says that the reflection will be minimum.[/QUOTE]

In order to get minimum reflectance, the path difference between the directly reflected wave and that, which reflects from the glass-layer interface has to be odd number times half of lambda(coating) .

3. Dec 3, 2014

### BvU

Destructive interference occurs at $2d =\lambda/2$, but also at other path differences....

4. Dec 3, 2014