how can interference occur within diffraction? My book says : 
The number of interference fringes occuring in the broad diffraction peak depends on the ratio d/a that is the ratio of the distance between the two slits to the width of a slit. In the limit of "a" becoming  very small, the diffraction pattern will become very flat and we will observe the two slit interference pattern.
How can interference pattern be observed in a diffraction pattern? Why does it depend upon the ratio d/a? Why will we observe interference when a is very small?
 A: Interference is the sum (even with negative sign) of energy or impulse of two water or sound waves at a given point. For photons this could not be applied since photons do not interact with each other at the energy level of our usual used light sources.
If, and only if, one agree that light is a stream of photons, the phenomenon of interference is not applicable to the intensity distribution behind edges. If one use the imagination of light as a waves this lead to an other complication. Youngs sketches, showing the interference pattern of water waves , are "frozen" pictures of moving patterns of interference maxima and minima. In our time of animated sketches (or videos) this can be seen clearly.
Furthermore one can show, that the photons  get influenced at sharpe edges and than spread out in such a way that they form stationary intensity patterns on an observation screen. The influence happens between the surface electrons - their electric field is concentrated on sharpe edges - and the electric field component of the photons. This point of view allows to explain even single photon experiments and even with a single edge instead of a slit or multi slits.
Last not least it has to be explained, how the EM waves get in phase at the edge. The non mainstream answer is, that the surface electrons of the edge(s) and the electric field component of light influence a common field and this quantized field deflect the light into intensity distributions. The pattern of this distribution is an image of the quantized field around the edge.
I'm here to learn and it would be nice to get responses around the first step, where I'm violating the description of physical processes.
