Why does Harper Lee put Atticus under the light bulb in front of the jail and much later in the novel under the streetlight?
I agree with the previous answer.
Atticus also represents the light of truth. He knows the truth and simply sits under the light waiting. He is stragecially situated near the newspaper office. Atticus is in the open space in plain view. The truth should be exposed and in plain view. Atticus under the light is Harper Lee's symbolism of "the truth" of this story.
Harper Lee uses the light to focus the reader on the darkness of the crimes and false accusations against Tom Robinson. If anyone were to attack Tom, he would have to go through Atticus and through the light.
It was the light that allowed Scout to recognize a familiar face. Once that occurred, the lynch mob dispersed. The light also allows Atticus' friend, Braxton Bragg Underwood, to cover him with a shotgun the whole time the mob is there.
Good question. Lee puts Atticus under a light in Chapter 15 for a variety of reasons. First, as a bit of character development/comic relief: Atticus is sitting in front of the jail reading under that light. That's a sign of what kind of man he is. More importantly, though, the light allows the children to see him keeping vigil, and forces the crowd of men from the town to show their faces. They can't do evil under the cover of darkness; they must step forward and be recognized. This is both literal and a metaphor for the court case, which exposes a lot of potentially negative elements of the town.