Double slit and quater wave plates

[PenKnight]

Just a bit confused/ need confirmation of what's happening.

We shine light at the double slit and see interference patterns. Place two opposite 1/4 waves plate, so that we have which way information, in front of the slits. This causes the pattern to disappear.

1. Under what condition does interference happen? Same polarization and wavelength?
2. Somehow the 1/4 waves plates stop inferences.

Is the pattern observe just the sum of the single diffraction patterns? Because their polarization changed, therefore can't interfere with each other. I can't recall if there was any patterns on the sides of the central maxima.
The pattern is just showing the intensity but not the interferences? That's interesting. Due to the way we detect the light. But if they don't interfere, what causing the no light action on the sides?
I think the circular polarization is killing me.

Have I just answered some of my own question?

Thanks
PenKnight

 

[azntoon]

1. Yes, interference can happen when the light waves have the same polarization and wavelength.2. The 1/4 wave plates change the polarization of the light waves, which means that they no longer have the same polarization and wavelength, and therefore cannot interfere with each other. The pattern observed is just the sum of the single diffraction patterns, since the light waves are no longer interfering with each other. There may be some faint intensity on the sides of the central maxima, but this will be much less than the central peak. The circular polarization is likely confusing you because it is a different way of looking at how the light waves interact with the double slit and 1/4 wave plates. Instead of looking at the polarization of the light waves, circular polarization looks at how the light waves rotate as they pass through the double slit.