Strange Optical Effect: See CD Fully Visible & Ruler Bulging

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Discussion Overview

The discussion centers around an optical effect observed when a CD is held in front of a ruler under an ordinary lamp. Participants explore the nature of the shadows cast by the CD and ruler, particularly noting the visibility of the CD and the bulging appearance of the ruler's shadow. The inquiry seeks to identify the name of this optical phenomenon and understand its underlying principles.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that the effect may be due to the outer edge of the CD being transparent, causing light to refract and blur the shadow.
  • One participant proposes the term "bulging of contact shadows" and references Leonardo Da Vinci as an early observer of this phenomenon.
  • It is noted that the effect is related to the size of the light source, which is extended rather than point-like, leading to blurred shadows.
  • Another participant mentions that moving the ruler further from the screen may enhance the effect due to increased blurring of the shadow edges.
  • Some participants express uncertainty about the exact nature of the shadows, with one noting that the shadow of the ruler appears sharp despite the bulging effect.
  • A link to the "Black drop effect" is shared, with a suggestion that it may relate to the observed phenomenon, though participants remain cautious about drawing direct parallels.
  • There is a discussion about the non-uniform luminance at the edge of the light source and its potential connection to the observed optical effect.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the name of the optical effect or its precise explanation. Multiple competing views and hypotheses are presented, indicating that the discussion remains unresolved.

Contextual Notes

Some limitations include the dependence on the definitions of terms like "bulging" and "contact shadows," as well as the unresolved nature of the mathematical or physical principles underlying the observations.

tom_
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In the picture a CD is held in front of a ruler. The light source is an ordinary lamp. You can see a shadow of the CD on the ruler. Remarkable is the shadow on the screen behind it, because you can see that the CD is fully visible and the ruler bulges instead.

Does anyone know this optical effect and if so, what is its name?
 
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tom_ said:
Summary: Does anyone know this optical effect and if so, what is its name?

View attachment 246564

In the picture a CD is held in front of a ruler. The light source is an ordinary lamp. You can see a shadow of the CD on the ruler. Remarkable is the shadow on the screen behind it, because you can see that the CD is fully visible and the ruler bulges instead.

Does anyone know this optical effect and if so, what is its name?
Is it because the outer edge of the CD is transparent? The light refracts there so blurs slightly.
 
On the name I'm not sure, "bulging of contact shadows"?
i remember the first (?) account of this phenomenon is from Leonardo Da Vinci.
The effect is related to the dimension of the light source, not point-like, but extended. So the shadows are blurred and not sharp. Getting closer the two blurred edges of shadows will merge and darkens, giving the effect of extending. It's a spatial convolution of shadows.
If you move the ruler more distant from screen the effect must increase, as the blurred edge will be more strong, as the cd is.
 
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pinball1970 said:
Is it because the outer edge of the CD is transparent? The light refracts there so blurs slightly.

The experiment was not performed by me. I just was not able to give a reasonable answer, so I asked here.

He means, that the CD is completely metallized. The light source is the sun. Furthermore he writes that the shadow of the first object is always moving inboard and the shadow of the second object that is nearer to the image plane gets stretched outboard.
 
effed3 said:
On the name I'm not sure, "bulging of contact shadows"?
i remember the first (?) account of this phenomenon is from Leonardo Da Vinci.
The effect is related to the dimension of the light source, not point-like, but extended. So the shadows are blurred and not sharp. Getting closer the two blurred edges of shadows will merge and darkens, giving the effect of extending. It's a spatial convolution of shadows.
If you move the ruler more distant from screen the effect must increase, as the blurred edge will be more strong, as the cd is.

Could it be this?: https://www.researchgate.net/deref/https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FBlack_drop_effect

I noticed that the shadow of the ruler is actually very sharp. Nevertheless, you can see how it bulges out, as if the light rays that pass directly the edge of the CD are slightly bent towards the CD (The experiment was not performed by me. I just was not able to give a reasonable answer, so I asked here.)
 
tom_ said:
Could it be this?: https://www.researchgate.net/deref/https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FBlack_drop_effect

I think yes, it's related to the darker limb of sun, the non uniform luminance at the edge is the same factor is as here. (But thinkong better it's not a convolution as i wrote..)
Leonardo report this effect when standing in the light of sun in the frame of a open window, giving the back to sun, and looking at the shadow on the floor, moving far the window the shape of the shadow of the head will bulge toward the edge of the shadow of the window as they are close to contact.
 

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