Newton's Rings central spot brightness

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SUMMARY

The discussion centers on the phenomenon of Newton's Rings, specifically the contrasting brightness of the central spot when illuminated from above versus below. When light is shone from below, the central spot appears bright due to constructive interference, while illumination from above results in a dark central spot due to a π phase change caused by reflection at the glass-air interface. The thickness of the plano-convex lens also influences the visibility of the central fringe. Understanding these principles is crucial for analyzing optical interference patterns.

PREREQUISITES
  • Understanding of optical interference principles
  • Familiarity with plano-convex lens properties
  • Knowledge of phase changes in light reflection and refraction
  • Basic concepts of refractive indices
NEXT STEPS
  • Research the principles of optical interference in detail
  • Explore the effects of lens thickness on interference patterns
  • Study the behavior of light at interfaces with different refractive indices
  • Learn about Newton's Rings experiment setup and analysis
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Students and professionals in optics, physicists studying wave phenomena, and anyone interested in the practical applications of interference patterns in optical systems.

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How come when the illumination is from below, the central spot is bright, but when the illumination is from above, the central spot is dark?
 
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Do you know what causes the rings?
 
I think when shining light from underneath, whether a central bright fringe is obtained or not also depends on the thickness/height of the plano-convex lens.

For the light above case, at the centre, the 2 rays travel the same distance, only that one is reflected off a medium (air) of lower refractive index, so no phase change. The other is refracted off a medium of higher refractive index (glass), so there is a pi phase change, leading to a central dark spot
 
Ibix said:
Do you know what causes the rings?
Hi sir, could you please explain why shining light from underneath produces a bright spot at the centre?
 
phantomvommand said:
Hi sir, could you please explain why shining light from underneath produces a bright spot at the centre?
Let's say you are on the other side of the glass from the light. The center of the curved and straight glasses touch. Will you get any reflection at the interface (hint: is it really an interface if two identical materials are in direct contact)?

If you get reflection, what effect does this have on the intensity of transmission? If you get no reflection, what effect does this have on the intensity of transmission?
 
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Ibix said:
Let's say you are on the other side of the glass from the light. The center of the curved and straight glasses touch. Will you get any reflection at the interface (hint: is it really an interface if two identical materials are in direct contact)?
This is an example of why I love PF so much. Just a few words and no equations to give pretty much the whole story!
 
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