Understanding OPD in Light Ray Equations

[kelvin macks]

Homework Statement

i don't understand why the OPD of ray 1 and ray 2 is as shown in the notes.i don't understand why the equation given is (nt +nt)-(0.5 lambda)... why is nt +nt ?

Homework Equations

The Attempt at a Solution

 

[Simon Bridge]

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

 

[kelvin macks]

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

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?

 

[Simon Bridge]

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?

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?