# Homework Help: Wavelength in the visible spectrum is most strongly transmitted through a thin film

Tags:
1. Apr 2, 2015

### xamy

1. The problem statement, all variables and given/known data
A thin film of oil (n = 1.50) is spread over a puddle of water (n = 1.33). In a region where the film looks red from directly above (λ = 626 nm), what is the minimum possible thickness of the film?

2. Relevant equations
2nt = (m+0.5)λ
t = (m+0.5)λ/2n

3. The attempt at a solution
t = (1+0.5)(626E-9m)/2(1.50)
t = 313 nm

Anyone please correct the solution.

2. Apr 2, 2015

### TSny

You are looking for the minimum thickness. Did you choose the correct value of the integer m to get the minimum thickness?

3. Apr 2, 2015

### xamy

I think it should be zero

4. Apr 2, 2015

Right.

5. Apr 2, 2015

thanks

6. Apr 2, 2015

### xamy

1. The problem statement, all variables and given/known data
A camera lens (n = 1.29) is coated with a thin film of magnesium fluoride (n = 1.71) of thickness 87.0 nm. What wavelength in the visible spectrum is most strongly transmitted through the film?

2. Relevant equations
2nt = mλ
λ = 2nt/m

3. The attempt at a solution
λ = 2(1.71)(87E-9m)/(1)
= 297.54 nm

Solution is not correct.

7. Apr 2, 2015

### Staff: Mentor

You failed to consider possible phase changes upon reflection.

8. Apr 2, 2015

### TSny

I think you have the phase change due to reflection accounted for. (You want maximum transmission and hence minimum reflection.) But is your answer in the visible part of the spectrum?

9. Apr 2, 2015

### xamy

No its not

10. Apr 2, 2015

### TSny

I might be overlooking something, but it doesn't appear to me that you can get an answer in the visible spectrum.

11. Apr 2, 2015

### Staff: Mentor

D'oh! You're right.

12. Apr 2, 2015

### Staff: Mentor

Oops, I merged the two threads after receiving a report that they were duplicate threads. I see now that they have slight differences in the problem statements. Sorry about that!

13. Apr 2, 2015

### TSny

A web search indicates that magnesium fluoride has an index of refraction of about 1.4. Glass (camera lens) is usually about 1.5. For these values you can get an answer in the visible.

Last edited: Apr 2, 2015
14. Apr 2, 2015

### Staff: Mentor

Looks like the problem statement (for the visible light question) was mistaken. Where is the problem from?