why it is impossible to observe the interference of 2 incoherent sources?

Answer 1

The rays from the two sources reach a point at any time, with different phases depending upon the distance between the point and the two sources.

The difference between the two phases at any instant will remain the same in the case of two coherent sources. The reason is that the phase difference is zero when they do start from the source.

The difference between the two phases at any instant will not remain the same in the case of two coherent sources. The reason is that the phase difference is not zero and they do vary with time , when they do start from the source.

Therefore in the former case interference is possible and in the latter case it is impossible.

Answer 2

An incoherent light source contains light of all phases at once, changing all the time. To get destructive interference, you need to have two light sources with steady phase, each 180 degrees out of phase with the other.

Phase in this sense means something like "the timing of the oscillations of the light wave".

Answer 3

If you want to see constructive interference (destructive ones are not very interesting...) the two beams must be identical.
it means that they have to be coherent in space, ie the same wavelenght and
in time, ie the value of the electro-magnetic field has to be the same, same frequencie and same phase (you can considere the phase as the angle of the electric fields oscillation with a reference vector wich is perpendicular to the propagation vector...... hope you understood)

Anyway, if any of those conditions aren't fullfilled you will never get any constructive interference.

Pratically, to have interference, we use one source and we divide the beam in two or more beams, and then we use them to create interferences

Answer 4

I think you either need to:
1.Clarfy the question
or
2.Ask this question in a specialized forum