What is the difference between Young's double hole experiment and Young's double slit experiment? I wanted to know why we usually prefer double slit experiment rather than the original double hole experiment, is there any difference between the observations of the two experiments?
 A: A circular aperture (a hole) produces a 2D (circular) diffraction pattern.  A larger hole gives a smaller pattern. If you stretch the hole vertically (making a slit), the pattern shrinks in the vertical direction to become a (1D) single slit pattern.  Combining two single slit patterns gives a double slit pattern and combining two circular aperture patterns would give a (2D) double hole pattern.
A: Although the total number of fringes in both the double slit and the double hole pattern remain the same, using slits instead of holes gives a nice intensity distribution pattern which is useful for demonstration purposes. The measurement of drop of intensity with angle per fringe as measured by a position dependent photodiode in a standard experimental setup consisting of the slits/holes illuminated by a laser is easier to fit using slits instead of holes as drop of intensity in directions perpendicular to the holes is not there in case of the slits and so unnecessary factors in curve fitting are removed.
A: First I would like to enlighten some few facts about light interference which are related to the question:
 Interference of light is the superposition of two monochromatic coherent light waves leading to the so called interference patterns , i.e.constructive and destructive interference zones called maxima and minima. If the light at the two slits is not monochromatic or not coherent you will only have the superposition of two light waves but with no conventional STABLE interference patterns."Coherence means the phase difference of the two wave sources does not vary with time". But how Young performed his original double slit (not double pin holes ) experiment in 1803 long before the invention of laser waves providing monochromatic coherent  light ? Simply he used sodium discharge lamp- monochromatic yellow  spectral line with the two slits being fed from one third slit short distance before the two. 
Back to the question why double slit interference experiment is much preferred to that of two pin holes ? The answer is that the width of the opening should be of same order  as the wave length WL of monochromatic light in order to perform interference.In this experiment the Monochromatic visible Light WL is 5890 A or 0.589 E-3 mm. The calculations y= n.WL .D /d goes the same for both cases with n=0 , 1 ,2 ..for  Central max,first , second maxima  ..etc and only difference is they will display, on the screen, either circular or rectangular maxima picturing the opening itself.
However,it is obvious that the rectangular two slits of width of the order of  0.01 mm width  and 10 mm length  would perform or display interference  patterns more efficient than two circular pin holes of diameter near to .01 mm
