[plebeian]

    I recently had a first-hand experience regarding the "shadow of a mountain" cast onto the underside of clouds.  As the sun was "setting" there were large clouds near the "horizon" that began to cast shadows onto flat cloud cover the was directly overhead.  I've really only come across light reflection as the explanation for this and was curious how one could further defend this or present a more solid theory as my observance of the phenomenon doesn't really support reflection as a viable solution.

[WellRoundedIndividual]

There was a previous topic that discussed this at length. I will try to find it and link it. But it came down to a disagreement (again) as to whether reflected light off of objects could reflect enough light to cast a shadow. My point was that to cast a shadow the reflection has to be specular. It cannot be diffuse. Although, the FEer (I dont remember who) correctly stated that snow can reflect visible and UV light at close to 90%, he failed to understand that it scatters the light and therefore is not bright enough to cast a focused light and therefore a resultant shadow from a mountain on to a cloud.

[stack]

There was a previous topic that discussed this at length. I will try to find it and link it. But it came down to a disagreement (again) as to whether reflected light off of objects could reflect enough light to cast a shadow. My point was that to cast a shadow the reflection has to be specular. It cannot be diffuse. Although, the FEer (I dont remember who) correctly stated that snow can reflect visible and UV light at close to 90%, he failed to understand that it scatters the light and therefore is not bright enough to cast a focused light and therefore a resultant shadow from a mountain on to a cloud.

Here's one of the more recent ones you may be referring to:

https://forum.tfes.org/index.php?topic=11555.0

[plebeian]

The main logical flaw in the reflection theory is that if it was actually causing the lighting under the clouds, and the shadow of the mountain, the shadow cast would appear close to the mountain.  As time goes by and the sun moves further from the mountain, the shadow would get farther from the peak.  The exact opposite occurs.  The sun casts the mountain's shadow to the ground and as the sun sets the shadow rises and eventually as the sun gets low enough is cast upward to the clouds.  That shadow then gets closer and closer to the peak as the sun sets beyond the horizon. 

[totallackey]

There is reason to believe any of the shadows could be the result of reflected light from the surface of the earth.

There is evidence all of these shadows could be cast by reflected light.

Shadows do not require specular or direct light.

Even moonlight is capable of casting shadows.

[manicminer]

So is Venus capable of casting faint shadows when at its brightest of mag -4.5 or very close to it.  Haven't seen that myself but I have seen photos of 'Venus' shadows.