Why isn there diffraction in each of the slits in Youngs two slit experiment?
Well, there is diffraction in the small slits, but the slit width is chosen to be small enough so that only the central part of the diffraction pattern is relevant. The first zero would be at a really large y. Also, for a small width the diffraction pattern looks basically spherical which is why they are approximated as spherical waves. It turns out that if the slit width is approximately equal to the wavelength of the incident light then this approximation can be made.
