In Shakespeare's Macbeth, Macbeth tells his wife about the witches in a letter. Act 1.5 begins with Lady Macbeth reading the letter aloud.
She begins the scene reading the word, "They," which is a pronoun that the reader understands refers to the witches. This means that she begins reading the letter aloud--and the scene opens--after she has already read some of the letter. Macbeth, presumably, tells her about the witches in the unread portion of the letter.
He also tells her more about the witches in the letter, which she continues to read aloud. The witches, however, are not the main topic of the letter--the predictions they made and the coming true of one of them are what the letter is really about. And Lady Macbeth's response to the letter, once she's finished reading, demonstrates this:
Glamis thou art, and Cawdor, and shalt be
What thou art promised.... (Act 1.5.15-16)
She's already determined that Macbeth shall be king as the witches predicted.
In addition to serving the dramatic/pragmatic purpose of introducing Lady Macbeth and informing her of the predictions, the letter also reveals more about Macbeth's personality. Macbeth reveals that he "burned" with desire to question them further--he is desperately ambitious. And he has already made the decision to trust them--something he wasn't at first so sure he should do. This will lead to his downfall.
In William Shakespeare's Macbeth, first performed in 1606, Lady Macbeth first learns about Macbeth's encounter with the witches in Macbeth's letter to her near the end of the opening act. Lady Macbeth reads the letter aloud, and reports, "They met me in the day of success, and I have learned by the perfectest report they have more in them than mortal knowledge. When I burned in desire to question them further, they made themselves air, into which they vanished" (1.5.1). The agents depicted here must be the witches, for they met with Macbeth on his "day of success," or on the day of his military victory against the traitor MacDonwald. They also possess more than "mortal knowledge," or the knowledge available to natural beings, and they leave the battleground by vanishing into thin air. With this letter, Lady Macbeth learns about Macbeth's encounter with the witches, and Macbeth does indeed inform her of their presence.
For more information, please explore the eNotes guide to this great play linked below!