Light in A Streetcar Named Desire represents the truth. Blanche spent as much time as she could in the dark and only when she was in the light did the truth come out. She even got a paper lantern to cover the naked light bulb, much like she was trying to cover the naked truth. She also only went out with Mitch when it was dark outside, refusing to let him see the truth about who she really is.
At the very beginning, she complains about the light when she stated "I won't be looked at in this merciless glare!" This implies that she instantly starts lying the second she walks into the room. She instantly begins to lie. She then puts all the blame of losing Belle Reve on Stella, saying "you left! I stayed and struggled!"
The truth finally comes out to Mitch in scene 9. Mitch says "I don't think I've ever seen you in the light... That's a fact!" He then takes the paper lantern off the light, exposing Blanche. He immediately calls her out on her lies, firstly pointing out that she lied about her age. The truth, much like the light, finally comes out. "I don't mind you being older than what I thought. But the rest of it- Christ!... But I was a fool enough to believe you was straight." The truth about Blanche finally came out after the light was shined on her. She told the story of her late husband (whether it was true or not) and the truth caused her to lose her last hope: Mitch.
In Scene 10, the paper lantern is mentioned again when Stanley caught Blanche on her lie. She told Stanley that she was going on a cruise to the Caribbean with a millionaire from Dallas. She then proceeded to claim that Mitch came by with a box of roses to apologize and she turned him down Stanley let her finish her little tale and then called her out, saying that there was no millionaire and that he had been with Mitch all night, so there was no way Mitch could have brought Blanche roses. "You come in here and sprinkle the place with powder and spray perfume and cover the light bulb with a paper lantern... I say- Ha!-Ha!