I have read about the relation between the wavelength and the hole in the Young double slit exiperemnt but I am not sure how that causes diffraction. I only know of one case which is a bundle of light diffracting due to the optical density of a transparent medium. are there any other cases causing light diffraction? ( classical and quantum views are welcome as long as the distinctions are made )
Any wave which isn't infinite in size diffracts, like water waves or elastic waves. For example, think about water waves after a breakwater:
Right after the breakwater there is a region where there is a wave and areas where there isn't. On the boundary between these two, some water molecules are part of the wave and rise up, and some don't and stay still. The molecules attract each other and some of the energy in the wave spreads to the surrounding molecules, so the wave appears to diffract. This phenomenon is described in the solutions of the wave equation, so every wave that satisfies the equation will diffract by the same principle e.g. electromagnetic radiation, i.e. light.
Even a single photon going thru any size aperture will change its trajectory (diffract) due to an interaction with the aperture itself and other surroundings. I think the interaction is with the electrons in the material and it affects the wave function of the photon. But it is not predictable unless you observe many photons and then you can see the distribution.