# Thin Film Interference

1. Nov 29, 2009

### Geekio

1. The problem statement:

A wavelength (λ)550 nm light ray traveling thru a viscous medium n1= 1.46 strikes a a crystal n3=1.5 protected by a film n2=1.25.

2. Relevant equation:

How can I determine the min thickness of the antireflective coating used to minimize the reflection of the incident light?

3. The attempt at a solution
I think I can determine it by using
2t+0.5λ1+0.5λ3= (m+0.5)λ2 where m=1 and then solving for t.

Any guidance would be greatly appreciated.

2. Nov 29, 2009

### Delphi51

Welcome to PF, Geekio.
I waited an hour and a half to post because I just don't understand this question.
To prevent reflection, you normally use a coating with index of refraction midway between the two media. In this problem your media are n = 1.46 and n = 1.5 while your coating is 1.25. At the 12 boundary your index is decreasing while at the 13 boundary it is increasing, so your reflections will be out of phase half a wavelength for starters. Oh, I think I see it . . . if the coating is half a wavelength thick and traversed twice, that is a total of 1.5 wavelengths and you'll get destructive interference. Clever! Never heard of it before but it ought to work.

So the answer is 0.5 wavelengths for the light in n = 1.25. I guess you'll have to figure out the frequency of that light from the information given at n = 1.46, then use that frequency to calculate the wavelength at n = 1.25.

3. Nov 30, 2009

### Geekio

thank you very much

4. Dec 3, 2009

### hiwa_83

hi ...i prepared thin film samples by spray pyrolysis technique but thier surface arent smooth ..can anyone tell me how can i make it smooth ???plz