Refraction is the bending of a wave when it enters a medium where its speed is different. The refraction of light when it passes from a fast medium to a slow medium bends the light ray toward the normal to the boundary between the two media. My question is what is the fundamental reason for this refraction?is it something related to the atomic force?
Refraction of light can be derived using classical electrodynamics. When light passes from one medium to another, its speed changes according to propagation constant. The frequency of light cannot change, so to make up for the changed speed, the wavelength changes. If you apply the boundary conditions, you get snell's law of refraction - refer to a classical electrodynamics book. See also Snell's law.