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?
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$\begingroup$ Prefer for what? Teaching? A slit gives a much brighter image, so it's much easier to demonstrate in the classroom to a large number of students. $\endgroup$– CuriousOneCommented Dec 25, 2015 at 11:55
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$\begingroup$ So there is no difference between any observations like the pattern of fringe etc? $\endgroup$– CuriousoneCommented Dec 25, 2015 at 11:58
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1$\begingroup$ The pattern is different. I don't think the total number of fringes varies, but I could be wrong. From an experimental point of view a slit simply allows a much larger amount of light to pass trough, so it's easier to see the pattern. With a bright laser one could, of course, illuminate a hole pattern nicely, too, but it's fun to do the experiment with white light to demonstrate that there is no need for a coherent light source. And if it's performed with a yellow sodium discharge lamp then there isn't that much light, to begin with. $\endgroup$– CuriousOneCommented Dec 25, 2015 at 12:01
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$\begingroup$ Okay I got your point,but you mentioned that white light will act as an incoherent source but won't the two slits act as coherent sources as any abrupt change in original source will reflect in the two slits acting as source? I came across this question that if you replace the monochromatic light with visible light you ll get a fringe pattern of different wavelengths overlapping incoherently.Why would they overlap incoherently? $\endgroup$– CuriousoneCommented Dec 25, 2015 at 13:45
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$\begingroup$ Yes, that's one of the things that the double slit experiment can teach: spatial vs. temporal coherence. A laser would be a spatially and temporarily coherent source, a white light is made spatially coherent but is still not coherent in time and that's enough for interference. There are variations on this theme, like using a white point light source to illuminate a dusty or soapy mirror. The reflection from the front and back together with the spatial coherence will cause very nice color fringes. $\endgroup$– CuriousOneCommented Dec 25, 2015 at 22:31
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3 Answers
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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.
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1$\begingroup$ I would like to clarify that the diffraction of two points actually generates hyperbolas (since the difference in the distance between two points remains constant). When the distance between the two wavelengths is close to a few wavelengths, fewer fringes will be seen but more curves, whereas if they are separated by many wavelengths, the hyperbolic fringes will appear almost straight. $\endgroup$ Commented Feb 25, 2021 at 13:11
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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.
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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