Effect of Partial Covering on Lenses and Mirrors

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Discussion Overview

The discussion revolves around the effects of partially covering lenses and mirrors on the illumination and visibility of images produced. It explores both theoretical implications and practical observations related to optics, specifically focusing on concave and convex mirrors as well as lenses.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants assert that covering a lens reduces the amount of light passing through, leading to a less illuminated image.
  • Others propose that covering a mirror may not only reduce illumination but also limit the "field of eyesight," affecting the number of viewing angles available to see the object reflected.
  • One participant discusses the role of interference effects, suggesting that the resolution of the observer and the characteristics of the covering material can influence the perceived image quality.
  • A distinction is made between real and virtual images, with some arguing that for real images, all parts of the mirror contribute to the image, while for virtual images, only the visible part of the mirror reflects light to the observer.
  • Examples are provided, such as the behavior of plane mirrors and magnifying glasses, to illustrate how blocking parts of these optical devices affects image quality differently depending on the type of image produced.
  • Concerns are raised about diffraction effects if a significant portion of the lens or mirror is covered, which could complicate the expected outcomes.

Areas of Agreement / Disagreement

Participants generally agree that covering optical devices affects illumination, but there is no consensus on the extent of this effect or the implications for visibility and image quality. Multiple competing views remain regarding the impact of covering on real versus virtual images.

Contextual Notes

The discussion includes various assumptions about the nature of light, the characteristics of the covering material, and the specific configurations of the lenses and mirrors being discussed. The effects of diffraction and interference are also noted as potential complicating factors.

Chen
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It's a known fact that when you partially cover a lens (with fabric or something), the image on the other side would look less illuminated, since less light rays go through the lens.

However, what happens when you partially cover a concave/convex mirror? Our teacher thinks the same thing happens, the image looks less illuminated. I, and some other pupils, think that by covering the mirror you reduce the "field of eyesight", i.e the number of places from which you can fully see the object in the mirror.

Can anyone confirm either claims, and explain why? :smile:
 
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We agree that for both mirror images, less light is apparent in general. Depending on the resolution of the observer, say on the scale of the rods and cones in his eye corresponding to the effective diffraction of the cloth and to (well-ground) mirror magnification, constructive or destructive interference can take place.

The interference depends upon the wavelength of light, the size of the cloth aperture, distance and angle to the observer, and whether or not he is involved classically. One analogy (not immediately obvious) might be two polarizers oriented either perpendicular or parallel to each other, with an intermediate fine cloth grid of arbitrary orientation.

That the mirror is well ground helps determine the coherence of interference. If the cloth is less than a few wavelengths from the mirror, the interference effect may be greatly distorted.
 
When you say you are covering part of the mirror with cloth, do you mean you put cloth on a fraction of the mirror, or you cover the whole mirror, but the loose weave of the cloth let's some light through?

Njorl
 
Originally posted by Njorl
When you say you are covering part of the mirror with cloth, do you mean you put cloth on a fraction of the mirror, or you cover the whole mirror, but the loose weave of the cloth let's some light through?

Njorl
I mean covering part (like half) of the lens/mirror with cloth that won't let light get through. I noticed I had misled Loren Booda, I'm sorry.
 
Last edited:
's OK. An interesting thought experiment, anyway.
 
Chen said:
However, what happens when you partially cover a concave/convex mirror? Our teacher thinks the same thing happens, the image looks less illuminated. I, and some other pupils, think that by covering the mirror you reduce the "field of eyesight", i.e the number of places from which you can fully see the object in the mirror.
An interesting question, Chen!

Here's my take: I think it depends on whether the mirror produces a real image or a virtual image. For a real image, all parts of the mirror carry light that contributes to the image. For a virtual image, only the piece of mirror that you are actually looking at reflects the light that hits your eye.

Check out an ordinary plane mirror. The image is virtual and clearly isn't affected by blocking off half the mirror. Also check out a magnifying glass, which also produces a virtual image (in normal use). Blocking off half the lens doesn't matter. But if I use the magnify glass to form a real image (like of the sun when you wish to start a fire), covering up half the lens will degrade the image (and the amount of light that is focused). (I used these examples since that's what I have handy.)

Of course if you block off so much of the lens/mirror that diffraction effects dominate, all bets are off. :-)
 
Thanks Doc Al, I was thinking along the same line. I'll speak with my teacher and let you know what she says (I'm fairly certain we are right though).
 

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