How can I observe the photons in the double slit experiment? In the double slit experiment with photons, the wave will collapse if I can observe which photons are going through which slit, right? Now, how can I do the observing? Does the polarization of light count as observing? If I place polarizing filters(ex: 3D glasses) in different directions(ex: a horizontal filter for the left slit, a vertical filter for the right slit) for each slit, will that detect which photon is going through which slit? Will I be able to see two bands of light on the screen? If not, is there any other way I can observe the photons?
 A: First of all, you can't "observe" photons. They are the 'entities' which help you observe things like a ball or an electron, but you can't use the same thing to observe itself primarily because a photon isn't a 'thing'. So, you can just observe the effects or collect the scattered photons from the screen. If you want to differentiate, the best option (according to me ) is to have two separate but close sources of monochromatic light, say green and red. Now with the image you get at the screen, you can assume what came from where. Polarisation may affect, but will surely affect the intensity. This method is free of all sorts of complexities.
A: If you put polarization filters as described you will not see interference picture on the screen. Because potentially you can detect polarization of the photon hitting the screen and hence know which slit the photon went through.
Frankly speaking I do not understand your last question about how photon can be observed.
The following attempt to deceive the double slits impressed me very much. When photon goes through the upper slit it pushes the screen with slits "up", when it move through lower slit - it pushes the screen "down". So let's measure the speed of the screen after photon passed through one of the slits and we will know which slit it went through!
Sounds doable, but. We do not know momentum of the screen exactly. If we know the momentum of the screen to be "exactly" zero (well, almost exactly), we can not know it's position and can not know which slit is where!
Whatever you construct "they" will always find out a way to explain to you why doesn't it work :)
