# Interferometer and Interference.

1. Feb 10, 2013

### binbagsss

I have some questions regarding a fabry perot interferometer. (they aren’t problems as such however I have had a good look around the internet and am struggling to find what I’m looking for).

1) Why is it that a circular fringe pattern is observed. How does a single source interfere with itself and is there variation in the pattern for a given wavelength as the distance between the plates is varied.

2) Am I correct in thinking that the term fringe visibility can only be used when the source is not monochromatic – e.g if there are two wavelengths present, at a given distance between the two plates, two independent ring systems are produced and fringe visibility refers to whether the bright and dark bands of each system coincide or not, and this is dictated by the distance between the plates. But then my next question is…

3) If the source is monochromatic and so changing the distance can not influence the fringe visbiity, what oservable affects does it give arise to?

Thanks a lot for any assistance on this. Really Appreciated.

2. Feb 10, 2013

### Simon Bridge

Interference can happen from a single source if there are at least two paths that light (or whatever) could be travelling. The interference comes from the phase difference between the paths.
http://en.wikipedia.org/wiki/Fabry–Pérot_interferometer
... see the diagrams of the possible paths.

http://en.wikipedia.org/wiki/Interferometric_visibility
I'm reading that as a strict "no": for some interference patters the intensity is not zero at every minima.
This happens when the the interfering waves are not equally bright.

In the fabry perot system - there is attenuation (due to reflection/transmission) in each path - so the waves which travel farthest will be dimmer.

Last edited: Feb 10, 2013
3. Feb 11, 2013

### binbagsss

From what I can see this does not explain why it is a circular pattern that is produced.

[/QUOTE]In the fabry perot system - there is attenuation (due to reflection/transmission) in each path - so the waves which travel farthest will be dimmer.[/QUOTE]

But does this have negligible influence on the ring pattern obtained, which is primarily caused by the different path lengths traveled by the light.?

4. Feb 11, 2013

### Simon Bridge

Circular lenses.
It does not change the pattern of light and dark fringes - it just affects the brightness of the fringes.
Imagine you have 2-source interference between a bright and a dim source - everything else the same. Where you have destructive interference to give a minima, the dim source will not be able to completely cancel the bright one.... but the minima are still in the same places. But if you changed the separation of the two sources, the pattern would change.

5. Feb 11, 2013

### binbagsss

6. Feb 11, 2013

### binbagsss

Also you mentioned the circular pattern being due to the shape of lens, however I have read on a few sources that if the plates are not exactly parallel the fringe pattern is not circular. Are the two connected?

7. Feb 11, 2013

### binbagsss

Sorry one more question, does the interference occur in the gap between the plates or once the rays have emerged?

8. Feb 11, 2013