# OPD of 2 light ray

1. Jun 8, 2014

### kelvin macks

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

i dont understand why the OPD of ray 1 and ray 2 is as shown in the notes.i dont understand why the equation given is (nt +nt)-(0.5 lambda)... why is nt +nt ?
2. Relevant equations

3. The attempt at a solution

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2. Jun 8, 2014

### Simon Bridge

could it be that ray 2 goes through the film twice.

3. Jun 8, 2014

### kelvin macks

do u mean the ray 1 has phase difference of -0.5lambda, whereas the light ray 2 undergo refelction twice so it should be 2nt. to find the relative difference between 1 AND 2 , just take 2nt -0.5lambda?

4. Jun 8, 2014

### Simon Bridge

Here, let me help with that:
Do you mean the ray 1 has phase difference of -0.5lambda, whereas the light ray 2 undergo reflection twice so it should be 2nt? to find the relative difference between 1 AND 2 , just take 2nt -0.5lambda?​

Consider: why should anyone be bothered to answer your questions if you cannot be bothered to type out the two extra characters in the word "you" (etc)?

Note:
"-0.5$\small\lambda$" is not a phase difference.
ray 2 does not undergo reflection twice - it is refracted at A and reflected at B.
So the short answers to your two questions are: no, and, no.

The effective path difference is computed from the actual distance traveled, modified by the phase changes.
What is the total physical distance ray 2 travels from point A to point B?