# Thickness between glass plates

1. Jun 7, 2005

### jetpeach

Hi, I generally understand interference and Newton's ring/Young's double slit experiments, but I can't find the answer to a pretty simple question:
When two glass plates are placed together and their is a gap of thickness "t", contructive and destructive interference leads to interference patterns as described http://physics.bu.edu/~duffy/PY106/Diffraction.html
but at what thickness will these patterns no longer be visible, at least by eye? Basically, I find tons of things saying, "from the inner and outer surface of a bubble light is reflected and interfers... and patterns show depending on m*lambda.... " But at what upper limit of the integer wavelengths of light does this stop? If you have two glass plates 1cm apart, clearly you don't see interference due to the gap between them, but what about 1mm? 100um?

Basically, one of my advisors states that because we can see interference patterns between our glass plates, they are separated by at most a few wavelengths of light, but I'm wondering why or how this is the case and if I can prove it. Thanks for any help! Be back in the morning to follow up,
jet

2. Jun 7, 2005

### shyboy

The presence of the interference pattern also depends on a radiation source. Because of reflections from different spots, the reflected waves reach an observer with a time delay. So the source should keep its phase during that time interval.
Usual non-laser sources have very short coherence time, so we cannot see any coherence if interfering waves arrives with a delay of several periods. Lasers, on the other hands, can keep their coherence longer, so we can see an interference pattern resulting from reflections from more separated surfaces. Now there will be another problem- to keep the distance constant with a precision better then wavelength.

3. Jun 7, 2005

### jetpeach

thanks

Ahhh, coherence time (and length) seem to provide be the answer to my question.
just if anyone else is interested