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?

 

[HalfLostAndSearching]

I can understand your confusion about the OPD (Optical Path Difference) in light ray equations. The equation given, (nt + nt) - (0.5 lambda), represents the total distance traveled by ray 1 and ray 2, which is equal to the OPD. The first part, (nt + nt), represents the distance traveled by the rays in the medium, while the second part, (0.5 lambda), represents the distance traveled in the air. This is because when light travels from one medium to another, it experiences a change in speed, which affects its wavelength and therefore its path.

To better understand this equation, it may be helpful to think of it in terms of phases. When two waves combine, their phases can either add constructively or destructively. In this case, the equation is representing the phase difference between the two rays, which is equal to the OPD.

I suggest reviewing the notes and any related concepts to gain a better understanding of the equation and how it relates to the OPD. Additionally, you can also try to visualize the path of the rays and how they interact with different mediums to gain a better understanding. I hope this helps clarify your confusion.