Strange Optical Effect: See CD Fully Visible & Ruler Bulging

[tom_]

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?

 

[pinball1970]

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.

 

[effed3]

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.

 

[tom_]

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.

 

[tom_]

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.)

 

[effed3]

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.