what happens if two glass slabs are placed in between the two slits..please explain me with their relative formulae...

Dear student,

We need the concept of alternating electromagnetic fields (waves) to explain certain physical phenomena, like the interference pattern in the 2 slit experiment. So we keep that. We must somehow explain how a particle orders of magnitude smaller than the distance between the slits somehow passes through both slits and interferes with itself. Problem, we cannot explain this well using "quanta" (Particles, photons). Wave mechanics gives a simple easy to understand explanation.

We need the concept of quanta (particles, photons) to explain other phenomena, like the Photoelectric Effect. So we keep that too. But, this means we use 2 different, mutually exclusive systems of mechanics to explain electromagnetic radiation.