In a room with a single light source on the ceiling, Keiji and Amina are standing at opposite corners. Between them, there is a large opaque cube on the floor, directly under the light. If Amina walks around the cube to approach Keiji while keeping her eyes on him, what happens to Keiji's visibility as seen by Amina?

Provide a very brief response.
As Amina walks around the cube to approach Keiji, there will be moments when Keiji is not visible to her because the cube will block her line of sight. The opaque cube creates a shadow region opposite to the light source, which Keiji is standing in. As she begins to walk, the cube will obscure Keiji until she comes to a position where the angle of her sight just clears the edge of the cube. As she continues to walk towards Keiji, he will become visible again once she moves out of the shadow the cube casts and out of line with the edges of the cube that directly block her view. She will maintain sight of Keiji as she walks further around the shadow and the cube until she reaches him.