# Path diffrence to produce destructive intefrence

1. Sep 7, 2014

### somecelxis

1. The problem statement, all variables and given/known data

A transparent flim of thickness t and reflective index n is placed in front of Y . Which one shows the new condition for destructive intefrence to occur ? the ans is (n-1) t = m λ
. In my opinion , it should be (n-1) t = (m-0.5) λ ,
because for destructive interference to occur , the path difference should be 0.5λ
2. Relevant equations

3. The attempt at a solution

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2. Sep 7, 2014

### BvU

You seem to be pretty lazy (which may be a good quality) not filling in 2 and 3. Never mind. Could you be so good as to post the problem statement in facsimile too ? Is it a multiple choice thing (since it asks "Which one shows.."). And how do you know the ans ?

3. Sep 8, 2014

### Staff: Mentor

Code (Text):

From what you've written, I surmise that the question may actually be along these lines.....

There is a point (which I'll denote as D) where there is a dark band. The piece of film is placed in front of Y. What is the condition for destructive interference to again occur at that point D?

4. Sep 8, 2014

### somecelxis

yes , this is roughly what the question means .

5. Sep 8, 2014

### somecelxis

In my opinion , it should be (n-1) t = (m-0.5) λ
Am i correct?
for destructive interfrence to occur , the phase difference must be (m-0.5)λ.
i.e. destructive interfrence occur at 0.5λ (180 °) , 1.5λ (540°) ......

6. Sep 8, 2014

### BvU

Could you be so good as to post the problem statement in facsimile too ? Is it a multiple choice thing (since it asks "Which one shows.."). And how do you know the ans ?

I think we can agree on that

7. Sep 8, 2014

### Staff: Mentor

No.
The total phase difference must be (m-0.5)λ, yes.

8. Sep 8, 2014

### somecelxis

since you said my (m-0.5)λ, is correct.... then why my (n-1) t = (m-0.5) λ is wrong?

9. Sep 10, 2014

### somecelxis

the other options are nt= mλ, (n-1) t = m λ , (n+1) t = m λ , and (n+1)t = 0.5mλ...
my choice is (n+1)t = 0.5mλ .... Am i wrong ?

10. Sep 10, 2014

### somecelxis

the other options are nt= mλ, (n-1) t = m λ , (n+1) t = m λ , and (n+1)t = 0.5mλ...
my choice is (n+1)t = 0.5mλ .... Am i wrong ?

11. Sep 10, 2014

### Staff: Mentor

There is not much point just guessing. It's time to draw some diagrams and think about this really closely.

Attach your diagrams so we can see how you go about it.

12. Sep 10, 2014

### BvU

well celxis, O2 has me puzzled too. I don't think there's a phase jump involved, and I also think the path difference is (n-1)t (didn't mean to create the impression that it isn't, in post #6), so his (her?) no in post #7 must have some other reason. something about m that we all take for granted ?

By the way, does the film have a reflective index n or an index of refraction n ?

And I do hope your + in the last line of post 10 was a typo. Stick to the minus sign...

13. Sep 10, 2014

### somecelxis

SORRY ， all。 please ignore my few post before it. my ans is still (n-1) t = (m-0.5) λ , but the ans is (n-1) t = m λ..
there 're few choices :
nt= mλ, (n-1) t = m λ , (n+1) t = m λ , and (n+1)t = 0.5mλ..

reason for my choice :
because for destructive interference to occur , the path difference should be 0.5λ