In William Gibson's play The Miracle Worker, we can recognize the symbolic use of light and darkness first in the stage directions as the play opens and the scene is set.
It is the middle of the night and Helen (the baby) has been very sick.
It is night at the Keller homestead. Inside, three adults in the bedroom are grouped around a crib, in the lamplight. They have been through a long vigil…
Among other things, darkness is often symbolic of evil or death. In this case, it symbolizes a lack of knowledge (on the part of the adults)—a reference we can recognize by the saying "to be kept in the dark." The light in the room comes from lamps that have been lit to dispel the darkness and allow the doctor to treat the baby. The light symbolizes the illumination of one's mind to truth. It is with the lamp before the baby's face that Mrs. Keller proves without a doubt what has happened to her baby.
As Mrs. Keller screams for him, Keller runs up the stairs to her.
KELLER: Katie? What’s wrong?
KATE: Look […] She can’t see. Look at her eyes.
(She takes the lamp from him, moves it before the child’s face.)
She can’t see!
Throughout the story, Helen lives in a world of darkness, not only literally because she cannot see, but also in that she is cut off from the world with no way to communicate.
At the end of Act Two, Annie Sullivan has convinced the Kellers to allow her to take Helen to the hunting cottage in an attempt to break through Helen's darkness—not her blindness, but the prison her blindness—and deafness—have created around the child.
At two o'clock in the morning (when it is dark), Annie decides to try to teach Helen, but Helen won't allow her teacher to touch her in order to sign letters into her hand. The darkness of the hour symbolizes not simply Helen's blindness, but also her lack of knowledge, Annie's frustration and perhaps even an ebbing hope that she will ever reach her student. Certainly the situation seems hopeless in that Annie cannot make any progress at the homestead with others interfering and coddling Helen regardless of her terrible behavior.
When Annie lights a match for the lamp, this symbolizes the coming of hope. Since Helen cannot abide Annie's touch, her teacher calls to Percy and places letters in his hand. Helen is curious but still resists Annie's touch. So Annie continues to spell to Percy until enraged, Helen bodily pushes Percy away:
(Helen now yanks their hands apart. She butts Percy away, and thrusts her palm insistently. Annie's eyes are bright with glee.)
Ho, you're jealous, are you!
(Helen's hand waits, intractably waits.)
(Annie begins to spell into Helen's hand, and Helen allows it.)
Good! So I'm finally back to where I can tough you, him? Touch and go! No love lost, but here we go.
At the play's end, when Helen and Annie are at the pump and Helen finally connects the word with the thing, light is used to symbolize illumination again. State direction notes:
And now the miracle happens. Helen drops the pitcher on the slab under the spout...She stands transfixed. Annie freezes on the pump handle. There is a change in the sundown light, and with it a change in Helen's face, some light coming into it we have never seen there...
It is at this point that Helen remembers and forces out the baby word she once knew for "water." In this the audience understands that Helen now knows that the words Annie has been teaching her represent things in the world around her.
Interestingly, the stage lighting in the story supports the symbolism of light and darkness. At the moment of Helen's awareness, there is a change in the stage's "sundown light."
Light is referred to yet again in Helen's face, symbolizing the light of knowledge.