What does observation mean in two-slit electron diffraction experiment? My question is clear, that I ask:
What do we mean by "observation" in 2-slit experiment for electrons (or any other wave-particle)?
You know, we say that :"if we observe the electron, it shows a particle-like behavior; and if we do not observe it, it shows a wave like behavior"
So, if the observation is through sending and getting photons, is it just through our eyes or through any other interruption by light beams?
I'm confused! Or if my sentences are silly, what's the correct approach to understand the term observation?
 A: Your question is not silly. In the two slit expariment you measure the position if the photon, that is a particle-like quality. What happens is that a photodetector records when it absorbs a photon. That gets recorded in the memory. Some schools of physics interpret the interaction of the photon with the detector as a measurement. Others interpret that the measurement only happens when a conscious being "reads" the measurement from the detector.
A: When you send the photons through the double-slited wall, they form a diffraction pattern on the other side, which is a wave-like phenomenon. It makes no sense to think of the photons as "particles" anymore, since they would need to cross both slits simoultaneously in order to create the diffraction pattern.
So a way to find out what's happening is to place photodetectors in the slits. But what happens when you place the photodetectors is that these absorb the photons which hit them, not allowing them to cross the wall. This means that we are able to find where the photon was going to cross, but we didn't let it cross the wall at all. The rest of the photons will cross the wall across the other slit, but they will not form the expected diffraction pattern.
But if we remove the photodetectors and send the photons trough the wall, even in tiny packets small enough to consider that we are sending them one by one, we get the diffraction pattern. This would mean a single photon is crossing both slits simoultaneously, which is definitely a wave property and not a particle one. 
I reccomend Feynman's lecture on the subject or the first chapter of Quantum Mechanics by Claude Cohen-Tannoudji.
A: A free moving electron does not emit photons. If one want to see an electron you have to illuminate it. Sending photons, the electron reflect (absorb and re-emit) photons which one can detect then. By doing this the electron get disturbed and changes his direction. The fringes on the observation screen will be destroyed. Since one cannot synchronize the electron's state with the photon's state the interaction between them always has an uncertainty and the result gets blurry. The uncertainty of each interaction (between the electron and the photon and between the resulting photon and the measurement instrument) is the reason for the impossibility to conclude about the position of the electron during the transition of an edge (or a slit or a double slit).
On the over side, the wave characteristic of the electron in the interaction with an edge (slit, double slit, diffraction foil) is an interpretation of some thing what is not observable. To be more precise when ever one observe the electron it is a particle with some uncertainty in the electron's state (velocity, position, momentum, ...) which one describe with wave function. That is very useful for calculations. Duality means that both features - wave characteristic and particle characteristic - coexist in every moment.
A: Double slit experiment with electrons is a demonstration of a quantum behavior. When we say observe we mean expose to some kind of interaction. So when electron travels from its source towards the double slit and then passes through, and hits the detector, we see that it is a particle. But, when many electrons pass through the slit we observe a interference pattern so we try to resolve this. Some people said its because electrons behave like waves but that would imply that every individual electron behaves like a wave and that was too strange. So people tried to resolve this by assumption that electrons interact in such a way to form this pattern...of course, new double slit exp was done, this time with only one electron at a time traversing to the slits and to detector and same pattern was observed. So, we have to conclude that free electrons away from interactions behave like waves and more strangely, that they pass through both slits at the same time. So, their position is not determined. There is some kind of probability that they are at some place and this probability wave travels to the detector and only there electron, because of interaction, becomes localized that is to say, nature assigns definite position to the electron. Observation therefore means some kind of interaction. 
