In Shakespeare's play Othello, how does the theme of wicthcraft relate to Desdemona and Emilia ?

Shakespeare’s tragedy Othello devotes
much of its first act to charges and denials of witchcraft. Brabantio, Desdemona’s
father, accuses Othello of having used witchcraft to charm Desdemona into marrying him.
Othello effectively refutes such charges by explaining the development of the mutual
love he shares with Desdemona. At the very end of the first act, it is Iago, if anyone,
who seems to be in league with the devil.

The first act’s
emphasis on witchcraft serves a number of relevant purposes, especially in helping to
characterize Desdemona and (later) Emilia.  The theme of witchcraft is relevant to these
two women in various ways, including the
following:

• Witches were generally considered
  evil figures with Satanic motives and Satanic connections. Usually witches were thought
  of as females, as in the opening scenes of Shakespeare’s Macbeth.
  References to witchcraft have the effect of highlighting, by contrast, Desdemona’s
  purity and innocence. She is so virtuous that she seems almost the opposite of a
  witch.


• Witches were often thought of as old and
  physically ugly, a fact that helps highlight, by contrast, Desdemona’s youth and
  physical beauty.


• Witches were often thought to operate in
  covens – groups of similarly malign figures. There are, for instance, three witches at
  the beginning of Macbeth, not just one.  The fact that Desdemona
  has such little contact with other women for most of the play (her only real female
  companion is Emilia) helps highlight her isolation and thus her vulnerability. She has
  no large group of female friends to rely on when Othello begins to become increasingly
  jealous and belligerent. Her only female companion is also her social inferior and thus
  is not in much of a position to help her in practical
  ways.


• Just as witches were often considered to be
  emasculating figures and could be viewed as projections of male neuroses, so Othello
  feels emasculated by Desdemona’s alleged adultery. Her supposed crimes are really
  reflections of Othello’s own overheated imagination.


• Just
  as witches were persecuted and often killed for crimes they did not commit, so the same
  thing happens to Desdemona.


• Just as witches were often
  slain by men who felt a special religious calling, so Othello sees himself as an
  instrument of divine justice when he kills
  Desdemona:

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. . . confess thee freely of thy sin . .
.

. . . Thou art to die. (5.2.53,
56)

• Just as
  witches were often accused of sexual transgression, so is
  Desdemona.


• Just as witches were often punished without
  recourse to genuine justice, so the same thing happens to
  Desdemona.


• Just as witches were supposed to operate with
  the assistance of other witches, so Othello accuses Emilia of assisting Desdemona in her
  supposed adultery by “keep[ing] the gate of hell”
  (4.2.91).

In short, the theme of witchcraft,
first introduced into the play by Desdemona’s own father, helps highlight, mainly
through contrast, many positive traits of Desdemona herself and also of
Emilia.