# 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. :)

• What do you mean by "the fringes are completely out of phase"? Do you mean that the pattern is shifted (so bright becomes dark etc.) compared with the pattern without any polarisers? – Philip Wood Jan 19 at 12:16
• @PhilipWood I fixed the terminology and added an example. With the 'erasing' 45 degree polarizer, when it is -45 degrees I get an interference patter, but when it is at +45 degrees I get another interference pattern that is shifted perfectly out of phase compared to the -45 degree interference pattern. – user1165664 Jan 19 at 16:15

• Note that polarizers don't have a direction (like a vector, $\rightarrow$ ), the have an alignment (like a tensor, $\leftrightarrow$), because they are unchanged by a 180 degree rotation. – JEB Jan 20 at 18:36