Why are my interference patterns completely out of phase? DIY physics enthusiast here doing a double slit eraser experiment at home with a laser pointer, double slit diaphragm, and few linear polarizers (horizontal at one slit, vertical at the other, +/-45 degrees for the eraser).
When I angle the eraser polarizer at -45 degrees or +45 degrees I get the interference pattern back, however the interference patterns (light/dark bands) are completely out of phase for -45 vs +45. How come that happens?
+45 =   |   |   |   |   |   |
-45 =  |   |   |   |   |   |
P.S. My physics "knowledge" is all from the University of YouTube, so you may have to explain it to me like I'm 5. :)
Edit: Here is a link to a video I uploaded showing the setup and shifting interference pattern I am talking about:
https://www.youtube.com/watch?v=1rN3iLcbb2M 
 A: The diagrams represent light coming out of the screen towards us. There are therefore oscillating electric and magnetic fields in the plane of the screen. We don't need to consider the magnetic field, so I've left it out. I'm assuming (to make things easy) that linearly polarised light from one slit has an electric field increasing in the upward direction while light from the other has an electric field increasing to the right. Hence the black arrows. [Soon the upward and right-ward fields will reduce and then become downwards and leftwards respectively, and so on. In other words the fields are oscillating.]
Polaroid filters let through only the electric field components oscillating only in one alignment. As you can see from the diagrams, the components of these fields that get through the analyser (eraser) polaroid will be in phase with each other when this polaroid is at +45°, but in anti phase when it is at –45°. The diagram shows the electric fields at a particular time, but these phase relationships continue to be true at other times.
So an extra half wavelength (or half a wavelength less) of path difference will be needed to give the same sort of fringe (bright say). Therefore there will be dark fringes where there were bright fringes previously, and vice versa.

A: The +45° polarised light has positive amplitude for both being vertically polarised and horizontally polarised, on the other hand -45° polarised light has positive amplitude for being vertically polarised and negative amplitude for being horizontally polarised, this difference makes the interference pattern on the screen out of phase.
