What is the minimum thickness for destructive interference in thin film?

[TwoEyedHuman]

1. Calculate the minimum thickness of the film for which the intensity of the reflected red ray is near zero.
(This is AP Physics B 2010 (c)ii.)
I found the wavelength in the film to be 4.81x10^-7 m
n_f=1.382. 2L=mλ (destructive interference)

The Attempt at a Solution

My first attempt yielded a result of L=2.41x10^-7 m
That was using the equation given. I checked the result in the scoring guideline, and it was wrong. I read the explanation, but I was confused by it and didn't want to write down an answer I didn't understand.

 

[Doc Al]

Can you state the complete problem.

 

[TwoEyedHuman]

Can you state the complete problem.

http://apcentral.collegeboard.com/apc/public/repository/ap10_physics_b_scoring_guidelines.pdf

Question 5

 

[Doc Al]

http://apcentral.collegeboard.com/apc/public/repository/ap10_physics_b_scoring_guidelines.pdf

Question 5

That's the scoring guidelines. Where's the problem statement?

 

[TwoEyedHuman]

My apologies.
http://apcentral.collegeboard.com/apc/public/repository/ap10_frq_physics_b.pdf

 

[Doc Al]

2. 2L=mλ (destructive interference)

What phase difference is required for destructive interference? What value of 'm' must you use in this equation?

 

[TwoEyedHuman]

What phase difference is required for destructive interference? What value of 'm' must you use in this equation?

That is where I get confused.

 

[Doc Al]

That is where I get confused.

Well, for two rays to destructively interfere--cancel out--what must be their phase difference?