I'm testing out an edge case in Cycles.
Here's the setup: I have two area lights: a large dim one and a small bright one. There's also a ground plane. Each light has a box behind it prevent light from shining from the back side. The large dim light emits a small amount of light downward and the small bright one emits light upward. The small one is positioned between the large one and the ground plane. Here's a diagram of the setup:
When I render the scene, I would expect that light from the small light should reflect off the large light and onto the ground plane below. However, that's not the case here. When I change the brightness of the small light, the illumination of the ground plane doesn't change at all. That suggests that there is no light being reflected off the large light source. Here are some example renders:
When I hide the large light, the light from the small one bounces off the object that was behind the large light and, as expected, illuminates the ground plane below. Some examples:
So my question is: why don't Emission BSDFs reflect light? In theory, they should; even the Sun reflects light (albeit a minuscule amount compared to what it emits). Is this a design choice or a bug? If it's a design choice, then why?