The Dark Fringe

You can do all of this using water waves instead of light waves if you prefer. Create a pool with a barrier which has a narrow slit near the middle. Then make some waves on one side of the pool, and see which waves end up getting through to the other side and crashing on the far wall. You can think of the waves as individual points in the pool moving up and down. Each time any point in the pool oscillates up and down, it send ripples out in all directions. Then all of the surrounding points are waving up and down, and those end up also sending out ripples in all directions.

It doesn't make much of a difference whether there is a 2nd slit or not, if not many ripples are making it to a particular point on the wall, making a slit there is not going to change that. Not much light or water waves will get through a slit which you place near a dark fringe.

Whereas a lot of light or water waves will get through a slit you place near a bright fringe. I do not understand the thinking behind your 1st question. In what way do you think the 2nd slit "disrupts the out-of-phase waves and create new wavelets that would create a new interference pattern"? We can think of the illumination at slit 2 as either a individual light waves from different parts of slit 1 arriving with approximately the same amplitude but different phases, or b darkness resulting from the collective destructive interference of the waves in a.

In case b it is obvious that there will be darkness behind slit 2, but what about case a? The different parts of the 1st slit can be thought of as coherent sources Huygens' Construction. Each of these sources produces its own diffraction pattern behind the 2nd slit, similar to that in your 1st image.

Each diffraction pattern has its own phase - or more accurately, each point in the pattern has a different phase depending on which source created it. But just as the sources at slit 1 are out of phase and interfere destructively at slit 2, so the diffraction patterns behind slit 2 are also out of phase, and interfere destructively at the screen. Your 2nd question is answered in What happens to the energy when waves perfectly cancel each other?

Interference

These answers explain that energy which is "lost" at places of destructive interference "reappears" at places of constructive interference. You cannot have destructive interference at one place without having constructive interference somewhere else. So there is always conservation of energy. But these answers don't seem to explain why or how this happens. The explanation may be that waves are not objects themselves but descriptions of the transfer of energy or probability or something else.

This distinction is explicit in quantum physics, between wave-functions and the particles they describe. So individual photons do not annihilate at points where there is destructive interference of the wave-function.

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But interference also occurs in Newtonian physics - eg sound in air, ripples on water. If we were to examine the collisions between elements of air or water, we would see the mechanics by which energy is transferred away from or towards certain regions at which we say destructive or constructive interference is occurring.

The wave picture is a shorthand method of describing some more complex underlying mechanism. Although it makes some things easier to understand such as the appearance of interference patterns , it makes other things more difficult to understand such as the disappearance of energy from one point and its reappearance somewhere else. By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service , privacy policy and cookie policy , and that your continued use of the website is subject to these policies.

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The dark fringe area produced no light after the second slit. Please consider my two questions. Lambda 2, 4 11 It is not clear to me what we are seeing in the 2nd image, because you say that "the dark fringe area produced no light after the slit. The second image shows the second slit positioned into a deconstructive zone between two constructive areas.

The slit did not photograph very well. It is lost in the glare of the two constructive areas. Does the 2nd image show only the light emerging through the 2nd slit? I applied the above formula and reached an expression, but found that my answer was in fact incorrect not even in the options - it was an MCQ and this formula had been used in the solution:. The maths doesn't change. Physically, of course, this variable change is absurd, since you would now have a "zeroth" dark fringe, an absurd title, since the central fringe is bright in this case.

In YDSE, the central fringe is bright, let us leave this aside for the time being from the counting procedures. By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service , privacy policy and cookie policy , and that your continued use of the website is subject to these policies. Home Questions Tags Users Unanswered.

I applied the above formula and reached an expression, but found that my answer was in fact incorrect not even in the options - it was an MCQ and this formula had been used in the solution: Which of the above formulae is correct? The Dark Side 2, 5 19 Ritwik Ojha 6 3.