# Homework Help: Thin films interference

1. Aug 16, 2007

### tan-X1

After I read many articles about measuring film thickness by interference method.

I tried to calculate flow that papers but I can't

the equation is

d= (y1*y2)/(2*(n1y2-n2y1))

for an easy I approximate n1 =n2

so

d= (y1*y2)/(2*n(y2-y1))

y1 = 350 nm
y2 = 380 nm
n = 2.35

I want to find out "d"

it seems to be easy

but I always get d = 944 nm

unfortunately, it's wrong

d have to be 472

that means I forgot to divide 2 in this eq.

however, the eq use to be right.

I don't know what's wrong in my calculation.

2. Aug 16, 2007

### tan-X1

Ahh, I am wondering

I track this eq from original eq

2nd=my1

2nd=(m+1/2)y2

I am able to get

d=(y2*y1)/(4*n*(y2-y1))

from this eq. I can get the right answer

so, what's wrong in this eq.

d= (y1*y2)/(2*n(y2-y1))

???

3. Aug 16, 2007

### HallsofIvy

Please do NOT post the same question in more than one forum.

4. Aug 16, 2007

### Staff: Mentor

Why don't you explain what this equation represents and define the terms? What are n1 and n2? y1 and y2?

5. Aug 16, 2007

Sorry mate.