Describe Mitch's use of light to expose Blanche.

All of Blanche's lies and illusion are destroyed with the flick of a switch. Mitch illuminates her face and exposes her untruths.

[Mitch crosses to the switch. He turns the light on and stares at her. She cries out and covers her face. He turns the light off again.]
MITCH [slowly and bitterly]: I don't mind you being older than what I thought. But all the rest of it--Christ! That pitch about your ideals being so old-fashioned and all the malarkey that you've dished out all summer. Oh, I knew you weren't sixteen any more. But I was a fool enough to believe you was straight.