# Homework Help: Requirements for an observable pattern in young's experiment

1. Sep 15, 2015

### Andy123

1. The problem statement, all variables and given/known data

It's not in fact a question of homework but something I have been wondering but cannot get answers about the requirements for an observable pattern in young's experiment:

1. Polarisation: In my study, the book derived the intensity function of light assuming the light beams are polarised. But I wonder if the experiment will work if unpolarised light beam is used, or the light beams have different degree of polarisation exiting the two slits?

2. Coherence: Some information from the internet tells that "coherence" of light beam from the two slits are not actually important. But its the "spatial coherence" and "temporal coherence" that matter. I am confused by what these terms mean and cannot understand anything by referring the wikipedia.

3. Laser: Why is laser used mostly in the experiment? I understand that it is "more monochromatic" than the other light. (although I don't why) But besides that, what else matter?

2. Relevant equations

3. The attempt at a solution

That's all the requirements I can think of for the young's experiment. I tried to answer the above questions but I failed. Please help! Thanks!

2. Sep 15, 2015

### DEvens

https://en.wikipedia.org/wiki/Young's_interference_experiment

You need slits that are very thin and spaced a reasonable distance apart to make the formulas work. The slits must be in an opaque material.

You need a relatively bright source of light. Mono-chromatic works better, but Young used sun light. A laser is a very convenient source of mono-chromatic light, and it is also very bright so you can easily see it.

You need an opaque "box" to perform the experiment in. You want the light to come in only through the slits so you can see the pattern.

You need some nice measuring equipment to know how far apart your slits are, how wide each slit is, and to lay out your observation screen.

Some kind of way to record your results is good. These days a digital camera such as on a cell phone would probably be quite adequate.

3. Sep 15, 2015

### blue_leaf77

If the polarization of the light is position independent, interference fringes can still be observed. In order to understand this, you can think of an unpolarized light as a lightbeam whose polarization direction rotates randomly with time- But since the fringe structure is independent on the polarization orientation of the interfering beams (so long as both are polarized at the same direction), a stable and by-eyes-observable fringes can be observed. This issue is actually what Fresnel-Arago experiments dealt with hundred years ago.
As DEvens said, the effect of partial coherence can be minimized by making the slits narrow enough and not separating them too far.

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