Newton's Rings central spot brightness

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Discussion Overview

The discussion revolves around the phenomenon of Newton's Rings, specifically focusing on the brightness of the central spot when illuminated from different directions. Participants explore the underlying reasons for the observed differences in brightness based on the angle of illumination and the properties of the materials involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant notes that the central spot is bright when illuminated from below, while it is dark when illuminated from above, prompting questions about the underlying causes.
  • Another participant suggests that the brightness of the central fringe when light is shone from underneath may depend on the thickness of the plano-convex lens.
  • A participant explains that in the case of illumination from above, the two rays at the center travel the same distance, but one ray reflects off air (lower refractive index) without a phase change, while the other refracts through glass (higher refractive index) and experiences a pi phase change, leading to a dark central spot.
  • Several participants seek clarification on the reasons for the bright central spot when light is shone from underneath, questioning the role of reflection at the interface of the lens materials.
  • One participant emphasizes the importance of understanding whether reflection occurs at the interface when two identical materials are in contact and how this affects transmission intensity.

Areas of Agreement / Disagreement

Participants express curiosity and propose various explanations, but there is no consensus on the definitive reasons for the observed phenomena. Multiple competing views and questions remain unresolved.

Contextual Notes

The discussion includes assumptions about the behavior of light at interfaces and the properties of materials involved, which may not be fully explored or agreed upon.

versine
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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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