Tennessee Williams' play, A Streetcar Named Desire, contains important light and dark symbolism. Most important in this symbolism is the light bulb in Stanley's apartment.
Prior to Blanche arriving in Elysian Fields, the bare light bulb in Stanley's flat represented his desire to keep life real. He is described as a man's man (poker playing, bowling, working man). He has nothing to hide from. Stella, his wife, accepts him as he is (brutish, violent, and loving). The bare light bulb allows each of them to see the other for what they are. The light bulb also shows how they both accept their lives together.
Once Blanche comes to Elysian Fields the bulb must be covered. Blanche, an aging Southern Belle, fails to accept reality. She refuses to live in the present (as seen through the multiple flashbacks). For her, the stark light represents the truth she is not wiling to face. Therefore, the naked bulb must be covered-- to hide reality.
Over the course of the play, the naked bulb represents the truth of what has happened. Stanley has driven Blanche to completely breakdown. Through this, Stanley is able to convince Stella to commit her sister and get her out of her home. Stanley is able to retake the power in his home--the light bulb has been uncovered.