# Given 2 indexes of refraction, how do I figure out thickness of oil

1. Apr 16, 2013

### riseofphoenix

:(:(

2. Apr 16, 2013

### tiny-tim

hi riseofphoenix!

show us your full calculations, and then we'll see what went wrong, and we'll know how to help!

3. Apr 16, 2013

### riseofphoenix

Given:
noil = 1.16
nwater = 1.33
λviolet = 400 nm - Question 1: why did they give me this if i didn't have to use it?
m = 1 (first order) - Question 2: what do they mean by the term "first order"? I never understand :(
t = ?

Equation to use:

2noilt = mλ
t = mλ/2noil
t = (1)(400 nm) / 2(1.16)
t = 400/2.32
t = 172.4 nm

I did it based off of this:

I don't understand their explanation though :(:(:(

Particularly this part: "Since nair < nair < nwater, light reflected from both top and bottom surfaces of the oil film experiences phase reversal."

Question 3: What's phase reversal??
Question 4: And why is this considered "constructive interference"? :(

Last edited: Apr 16, 2013
4. Apr 16, 2013

### riseofphoenix

5. Apr 16, 2013

### tiny-tim

hi riseofphoenix!
you have used it!
it's like a diffraction grating … there are different angles at which there's constructive interference, and this is the first one

(basically, the difference in phase needs to be a multiple of 360° for constructive interference)
phase reversal is where the reflected ray is 180° out of phase with the incident ray

whether it happens depends on whether the refractive indexes are increasing or decreasing (you need to look this up, and study it!!)

if there is phase reversal, you have to add 180° to the phase difference before you decide whether it's a multiple of 360°