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While moving an object between a light source and a clean mirror surface, its shadow seems hardly visible (at least compared with a non reflecting surface).
Is it correct that the shadow is not visible ?
How do you explain this behavior ?

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    $\begingroup$ Shadows are the lack of light, not an entity of their own. Shadows are cast wherever the light from a source is blocked by another object $\endgroup$
    – Jim
    Jul 23, 2015 at 14:20

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Everything you see that does not emit light itself is due to light from an external light source being scattered. Most objects scatter light in all directions, so no matter where you position yourself, you will see the object. If something blocks the light from the source, less light will be reflected from a particular area. This area is the shadow. Since light may come from several sources (e.g. light scattered several times in the sky), the shadow usually doesn't appear completely black.

Shadows are cast on mirror surfaces. But a mirror only reflects light in one direction, so to see the shadow you have to position yourself at a very special place, namely the place where you see the light source in the mirror. Again, if there are other light sources, the shadow won't appear black, and since parallel light rays remain parallel after hitting the mirror, you will in fact see a mirror image of the shadowing object.

The figure below may help explain. Solid lines are sunrays, dashed lines are where there would be sunrays if there hadn't been blocked by the tree. If you lay a mirror in the shadow of a tree, you'll have to place yourself exactly where the line extended between the sun and the tree, extended to the mirror and out by the same angle, hits your eyes.

drawing

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  • $\begingroup$ Thanks for the nice figure. Following the definition in Jim's comment, "shadow" is an absence of light. If I look in the mirror as you describe I will see the unlit side of the tree. I guess it can be described as "in the shadow" too. That is also easy to imagine when thinking about "volumetric fog" with light beams and shadow volumes visible inside. . $\endgroup$
    – wip
    Sep 29, 2020 at 11:44
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    $\begingroup$ @wip I didn't know the term "volumetric fog", but I can see it's a 3D rendering technique. It's a good idea to help visualizing — also in thought experiments like this — where the light rays go. $\endgroup$
    – pela
    Sep 29, 2020 at 13:44
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No we can not because shadow is formed on opaque objects.

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