I think you can just argue by symmetry that the ray must retro-reflect.
By the symmetry you can argue that the ray hasn’t reach the sphere.
Lumifer suggested something very similar as you, but this retro-reflection is equally possible as the retro-shadowing. Which do you prefer and why? Can’t be both.
By the symmetry you can argue that the ray hasn’t reach the sphere.
Lumifer suggested something very similar as you, but this retro-reflection is equally possible as the retro-shadowing. Which do you prefer and why? Can’t be both.
Can you explain?