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We can observe when we cause a slight disturbance at two points on the water surface which is intially totally undisturbed , it will form water waves which would look like as shown in below image:

enter image description here

we can observe that there are constructive and destructive interferences at some places and also which lies in between these type of interferences (that is between fully destructive and fully constructive) . We notice that it doesnt need screen to show the interference effects at all , so why in YDSE we need screen to show the interference patterns, is it because we cant be able to observe the interference being happen in air or any other medium from our naked eyes?

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  • $\begingroup$ Have you looked at the dimensions of diffraction of light? Water is in centimeters. This may help , studyphysics.ca/newnotes/20/unit04_light/chp1719_light/… . In example 1 distance between slits is 0.096cm . particularly the very last paragraph is of interest. $\endgroup$
    – anna v
    Commented Mar 27, 2022 at 5:59
  • $\begingroup$ I don't really have an answer for you, but what makes you think that we wouldn't observe the same thing if we had a reliable way to trace a path of electrons after the slits? The screen was a necessity when it came to firing one electron at a time. Try doing the same with one water molecule at a time and tell me what you see. $\endgroup$
    – bleuofblue
    Commented Mar 27, 2022 at 6:00
  • $\begingroup$ @bleuofblue I think the question is about light, the classical double slit experiment $\endgroup$
    – anna v
    Commented Mar 27, 2022 at 6:02
  • $\begingroup$ I ask the same thing though, if we had a way to trace the path of photons after the slits, would we not notice the same pattern of interference as the water? What happens when the region after the slits contains a medium like smoke which could highlight a laser path? The screen to me is just a tool to measure whats happening at any set distance from the source. Plop a detecting screen into that water, and we would have interference no matter what distance from the slits $\endgroup$
    – bleuofblue
    Commented Mar 27, 2022 at 6:08
  • $\begingroup$ read the last paragraph in the link I give, you can see single slit interference using your fingers $\endgroup$
    – anna v
    Commented Mar 27, 2022 at 6:12

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The screen is used simply because it makes the interference effect easier to see and to record and analyse. As is the case with the picture of the interference of water waves which you included in your question, the light interferes with itself everywhere. The use of a screen allows you to see the interference along a straight line, measure the fringes at a fixed distance from the slits, photograph them, and so on.

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  • $\begingroup$ Can you give a explanation how the screen shows the pattern , i mean i know that screen will be having interference but i mean to say how the waves which are getting transferred there are making the colours in the screen ? Although i understannd your answer :) $\endgroup$
    – ProblemDestroyer
    Commented Mar 28, 2022 at 0:44
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You need the screen because you do not want to stare in the sun with your eyes even if you were doing it through a peep-hole. Imagine adjusting the size of the diaphragm (of the slits) to see if there is already enough light has already fallen on your retina.

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Did you know that a water wave in a single slit will NOT make an interference pattern ... but light will! Water and light are similar but also very different. In the DSE there is NO light in the dark areas .... photons will travel to areas that are more "resonant" (path length is a multiple of wavelength), this is quantum mechanics/optics.

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  • $\begingroup$ What makes you say this? If the slit has the right size compared with the wavelength it will produce a diffraction/interference pattern. You can see this in simulations and videos of ripple tank waves. $\endgroup$
    – nasu
    Commented Mar 28, 2022 at 4:41
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    $\begingroup$ @nasu you will see in many wave tanks that there are reflections off the edges which cause additional waves. It is a common misconception as the Huygens principle (which is best for matter waves) is incorrectly applied to light especially in high school. Huygens for light requires the source to be broken into many sources. In sound waves sources there are many reflections off edges as well .... but interferences in sound are also caused by walls etc. There is no interference for sound from a single slit if we are in open space. $\endgroup$
    – PhysicsDave
    Commented Mar 28, 2022 at 14:24
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    $\begingroup$ Also note that diffraction and interference are 2 separate concepts .... but they commonly are used to refer to both. $\endgroup$
    – PhysicsDave
    Commented Mar 28, 2022 at 16:06
  • $\begingroup$ @PhysicsDave This is correct but it's very hard to convince anyone. I have tried to find real images or videos that prove interference patterns form when water waves go through a single slit but there are none. $\endgroup$
    – Bill Alsept
    Commented Jun 4, 2023 at 22:52
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    $\begingroup$ @BillAlsept yes it's impossible .... the historical textbooks shy away from the concept .... briefly mentioning single slit diffraction and quickly jumping to 2 slits ... where the math can easily start. $\endgroup$
    – PhysicsDave
    Commented Jun 4, 2023 at 23:16

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