Physics Help Forum Newton rings and coherence

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 Apr 28th 2010, 04:32 PM #1 Junior Member   Join Date: Apr 2010 Posts: 10 Newton rings and coherence I'm trying to understand Newton's rings. So we have a plano-convex lens supported in a plane (please, see image here http://scienceworld.wolfram.com/phys...tonsRings.html). The incident light is divided into the light that is reflected at the convex surface and the light that is reflected at the supporting plane. These two waves interfere, at least close to the support point. Now what I want to understand is why we ignore part that is reflected at the plane surface of the lens. I think it has something to do with coherence. But I can't figure it out. If someone could explain this to me in clear terms, I would very much appreciate it. Thanks.
Apr 29th 2010, 12:05 AM   #2
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 Originally Posted by becko Now what I want to understand is why we ignore part that is reflected at the plane surface of the lens. I think it has something to do with coherence. But I can't figure it out. If someone could explain this to me in clear terms, I would very much appreciate it. Thanks.
To have interference, the coherent sources must be close to each other.Light reflected from the top and bottom curved surface of the plano-convex lens are coherent but they are far away from each other. So they cannot produce interference. But the rays reflected from the curved surface of the lens and plane surface of the glass plate are close to each other. So they can produce interference.

 Apr 29th 2010, 06:29 PM #3 Junior Member   Join Date: Apr 2010 Posts: 10 Trouble is I don't quite understand the concept of coherence. What I need is a simple explanation of what is going on in this Newton's rings example, without using the word coherence (since I don't quite know its meaning). Then I think I can begin to intuition the meaning of coherence. Thanks.
Apr 29th 2010, 10:26 PM   #4
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 Originally Posted by becko Trouble is I don't quite understand the concept of coherence. What I need is a simple explanation of what is going on in this Newton's rings example, without using the word coherence (since I don't quite know its meaning). Then I think I can begin to intuition the meaning of coherence. Thanks.
When you illuminate a double slit by a monochromatic light, the light coming from the slits is coherent source of light. They are identical in all respect, just like your image in the mirror.
To get coherent source, some how you have to spit single source into two sources. To have the interference, these two sources must be narrow and close to each other.

 Apr 29th 2010, 11:11 PM #5 Junior Member   Join Date: Apr 2010 Posts: 10 Ok. So how is this related to what goes on with Newton rings? I understand the light is not pure monochromatic, but quasi-monochromatic, which means it is a superposition of waves with very close frequency. And I know that this has something to do with coherence, and is the cause of the lost interference. What I need is an explanation of why the light reflected off the top surface of the lens doesn't interfere with the other waves. But without mentioning coherence (since I don't understand the concept), just in terms of the light waves and what is going on with them. Thanks.

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