# 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

• 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

## 1 Answer

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.

• Thanks for taking the time to respond to my question! I appreciate it! I'm not sure what I am looking at with your diagram. Either I'm not doing a good job explaining the setup or your answer is going over my head (probably the latter). I added a video link just to make sure to rule out the former. – user1165664 Jan 19 at 23:03
• Thanks for replying. Do you know what is meant by light being a transverse wave? If you do, I'll take it from there. – Philip Wood Jan 19 at 23:09
• After a quick google search, I think I got the meaning of transverse wave. – user1165664 Jan 19 at 23:25
• Right. Light is a transverse wave. What is doing the oscillating (at right angles to the direction of travel of the light) is called the Electric field. So in my diagrams, where the light is coming out of the screen towards you, the electric field is oscillating in the plane of the screen. Because the polaroids across either slit are at right angles, the electric fields in the light from the two slits are at right angles. The fields are represented by the black arrows. The red arrows are the components of the electric fields that get through the analyser (eraser) polaroid. Alright so far? – Philip Wood Jan 20 at 8:54
• 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