vijayantv said:
but after the two slit it is splitted into two wave and and then the collision of two is causing interference pattern. is that not correct?
No - that is not a useful way of thinking about it at this stage.
For instance, there is nothing to "collide" - it's just maths, probability amplitudes, no substance there at all.
In the maths, which you want to avoid, the probability amplitude is a complex number - it involves the square-root of minus one.
Perhaps we need to be careful...
The setup is as follows:
We define a x-y coordinate system in a plane.
We put a pair of slits width a at the origin so one slit is centered on y=+b/2 and one slit is centered at y=-b/2. We need b>a to get two distinct slits.
The source is placed on the -x axis at position x=-S
A detector may be placed anywhere - D=(x,y).
The theory tells us the probability that the detector will go off (detecting a photon) given the properties of the source along with the type and (x,y) position of the detector.
We can get cute and work out the probability p(x,y) that the detector will go off, sometime during the experiment, given that a single particle has been sent out from the source. This is worked out using the single particle wavefunction - and some other maths - and depends on the detector position (and the exact setup).
If n non-interacting particles are sent out, then the probability p
n(x,y) that at least one will hit the detector can be worked out from the single-particle case in the usual way for the probability of independent events.
The average number expected at the detector is np(x,y).
The probability function for such a setup does not change with time - it is static.
Now try to ask your questions within that framework.
A lot of them stop making sense. i.e. it makes no sense to talk about a probability wave passing through the slits because the probability function does not move about. You could picture it as standing waves if you like - that's close. We can decompose standing waves into two interfering traveling waves if we like ...
Things can get tricky for other situations - it depends on how much we control the photons that get sent out by the source and exactly how we build the slits. (This statement included because I know there are people here who have had a major cringe-reaction at the simplifications I just made.) Let's not deal with that right now - no entanglement, no funny polarizations, nothing like that. Let's get you familiar with just the basics.
I have left out the details of how we find the probability distribution before and after the slits ... that's maths. Instead I've concentrated on how we shift from the single particle case to the multi-particle case - for the same setup.