Note the opening description of Blanche. She is in her old dilapidated clothes — her last remnants of a past life. The "Varsouviana" music — the tune which played when her husband shot himself — is heard as background music and Blanche is drinking to escape it all.
The appearance of Mitch, unshaven and dressed in his dirty work clothes emphasizes again that he is Blanche's last chance — that he is a rough and rather uncouth character.
With Mitch's appearance, Blanche immediately begins to act the part of the innocent young girl and the polka music stops. But almost immediately she realizes that something is wrong and the music begins again. During the first part of this scene, Blanche talks so much that Mitch doesn't have a chance to make his accusations against her. Her incessant line of chatter functions to cover up her fears and to postpone hearing what she fears to hear.
Mitch's first confrontation comes when he forces Blanche under the light. This act has multiple significance. First, on the realistic level, Blanche has deceived Mitch about her age and the light reveals Blanche's deception. The revelation of this deception leads to the other deceptions. Second, Blanche has constantly avoided the light ever since her young husband shot himself. She has had nothing stronger than a candle light since his death. Thus, Blanche has passed her life in semi-darkness and to be forced into the light makes her violate her inner nature. Third, being forced into the light here symbolizes the revelation of the truth about Blanche's past life. She has tried to conceal her life of dissipation and when Mitch forces her under the light, it is the same as making her realize and confess her past life. And fourth, Blanche's whole theory of living involves magic and illusion. She doesn't want realism. Instead, she prefers the magic of illusion. And rather than the truth, she lives for "what ought to be." Thus forcing Blanche into the light makes her see things in their ugly realism — that is, it makes her see how her life actually was instead of how it ought to have been.
