# Physical Optics : Air Wedge - Interference

1. Oct 24, 2005

### chickens

Hi there, I was doing some past year paper of my coming exam...and I got the answers for the question too, but one part I'm totally blur and cant understand that well..... well here goes all the questions and solution:

The part that i'm confused is with the part (b)...the answer given by both different book .... the solution seems.....a little blur to me.

Unclear to me:

1. why "for the first time the two lights coincide, change in m = 1" ?
2. the long and madness working.....i totally have no idea what is going on...

Anyone kind here could help me explain all the steps? I'm very blur... thanks in advance.

2. Oct 25, 2005

### chickens

no one could help to explain ? :(

3. Oct 25, 2005

### Staff: Mentor

Do you understand the meaning of this equation?
$$2 t = (m + \frac{1}{2}) \lambda$$

There will be constructive interference (and thus a bright fringe) for values of m = 1, 2, 3, etc. You'll get a yellow fringe whenever a red and green fringe overlap. They can't overlap with both having the same m, so find the first point of overlap when $\Delta m = 1$.

4. Oct 25, 2005

### chickens

yes that equation is for the condition to get constructive interference.....but for the delta m....i know it cant have the same m, but cant the first point of overlap is when delta m = 2 or 3 or so on?

5. Oct 25, 2005

### Staff: Mentor

Sure, it could turn out that the first overlap happens at some higher difference. But the only way to find out is to try it and see. (You have to solve this problem semi-numerically.) Start with $\Delta m = 1$. If that provides a solution, you're done; if not, then increment to $\Delta m = 2$. And so on until you get a solution. Of course, this problem is a set up, so the solution appears with the first try!

6. Oct 25, 2005

### chickens

ahhh, lol a set up q :P thanks for the explaination.....

then what about the second solution? it seems simple but doesnt seem that right lol

7. Oct 26, 2005

### Staff: Mentor

error in second solution

I looked at the second solution. I believe it is incorrect. The mistake occurs when they say:
Not so: The first fringe occurs at half the separation distance.