Strange Optical Effect: See CD Fully Visible & Ruler Bulging

In summary: The experiment was not performed by me. I just was not able to give a reasonable answer, so I asked here.
  • #1
tom_
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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?
 
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  • #2
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.
 
  • #3
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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  • #4
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.
 
  • #5
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.)
 
  • #6
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.
 

Related to Strange Optical Effect: See CD Fully Visible & Ruler Bulging

1. What causes the strange optical effect of seeing a CD fully visible and a ruler bulging?

The strange optical effect is caused by the phenomenon of refraction, where light bends as it passes through different mediums. In this case, the light is passing through the curved surface of the CD and then through the air, causing the ruler to appear distorted.

2. Can this effect be seen with any other objects besides a CD and ruler?

Yes, this effect can be seen with any object that has a curved surface and is placed in front of a flat surface. For example, a glass of water in front of a straight straw can create a similar optical illusion.

3. Is this effect the same for everyone who sees it?

No, the perception of this effect can vary from person to person. Factors such as the angle of viewing and the individual's vision can affect how they see the optical illusion.

4. Are there any practical applications for this strange optical effect?

Yes, this effect can be used in the design of optical devices such as lenses and mirrors. It can also be used in art and photography to create interesting and unique images.

5. Can this effect be explained by any scientific theories?

Yes, this effect can be explained by the laws of physics, specifically the principles of refraction and the behavior of light. It is also related to the concept of visual perception and how our brains interpret the information received by our eyes.

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