# Finding thickness of a thin film question

1. Apr 17, 2007

### feelau

1. The problem statement, all variables and given/known data
A thin film of oil (n = 1.23) is located on a smooth, wet pavement. When viewed perpendicular to the pavement, the film reflects most strongly red light at 640 nm and reflects no light at 569 nm. How thick is the oil film?

2. Relevant equations
2nt=(m+.5)*(wavelength)------constructive interference
2nt=m* wavelength------------destructive interference
n=index of refraction
t= thickness
m= order number

3. The attempt at a solution
So I tried to set the two equations together and using the corresponding wavelengths to solve for m. Next I just plugged m into one of the equations and using the corresponding wavelength to find t. Is that the correct method of doing this problem?

2. Apr 17, 2007

### G01

I believe you have the formulas for constructive and destructive interference reversed somewhat. They should be:

Constructive: $$2nt = m\lambda$$

Destructive: $$2nt = (m-1/2)\lambda$$

3. Apr 17, 2007

### feelau

Hm I think they are the same thing. My text book says it's that way, the only difference is the 90 degree phase change. But would be method be correct?

4. Apr 17, 2007

### G01

Yes, I guess they are both correct. Just in case you were wondering I looked up the formulas for thin films posted above in Knight's Physics book.(Not my favorite, but the only one I had on me.) Sorry for any confusion I may have caused.

Last edited: Apr 17, 2007
5. Apr 17, 2007

### feelau

np thanks for the verification