Why is a slit needed in youngs experiment? Can two coherent light sources not undergo constructive and destructive interfernce without the need of a slit?
Slits create a convenient setup, but are not the only way to investigate the wave properties of light. In fact, Thomas Young did not use slits in his famous experiment.
"Strictly speaking, there was no double slit in the original experiment as described by Young. Instead, the sunlight reflected off a steering mirror passed through a small hole in a paper, and the resulting thin light beam was then split in half alongside a paper card."
"Thomas Young, speaking on 24 November 1803, to the Royal Society of London, began his description of the historic experiment. His talk was published in the following year's Philosophical Transactions, and was destined to become a classic, still reprinted and read today."
If you mean the first slit in the original Young's design
It is because in order for interference effects to appear the wave impinging on the double slits has to be coherent, i.e. the phases known. Best would be a plane wave, but at that time there were no lasers, as dmckee has observed in the comment, and the way to get a mathematically describable wave with phases known is passing an incoherent beam through a small slit so that a radial wave appears. There are details in the answers to this question here.
If you mean two coherent beams in space, the experiment has been done and yes two beams split off from the same laser, so as to have two coherent beams, do exhibit in space interference effects. See this youtube video .
The challenge is to have two light sources that are mutually coherent. Two separate laser sources would not produce an interference pattern, because, although they are separately coherent (each has a finite coherence length), they are not in general mutually coherent; there does not exist a fixed phase relationship between the two separate sources. To make them mutually coherent is a nontrivial task. One would have to lock one on the other, which in the end would imply a very complicated experiment compared to just having two slits.
protected by Qmechanic♦ Oct 13 '17 at 6:25
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