Sodium light on 2 glass bars -- What's going on here?

[tensor0910]

Homework Statement

This is less of a calculation and more of a observation problem:

Our teacher shined a sodium light on top of two stacked pieces of translucent glass. We then had to bend down to get eye level with the stacked glass. When we did this you could see dark ( concentric?) rings reflecting off of the glass...sort of like contour lines on a map. When we pressed the two pieces together the lines straightened out until they were all parallel. What is going on here?

Homework Equations

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It has something to do with thin film interference...
Maybe the equation 2T = (n + 1/2 )λ is involved

The Attempt at a Solution

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Im pretty lost here. Thanks in advance for any responses.

 

[BvU]

You might google Newton's rings

 

[tensor0910]

thanks for the tip BvU. I think I got it now.