How is the theme of appearance versus reality explored in "The Merchant of Venice"?

There are many ways in which the play explores this
question. One example occurs when Portia's suitors try to answer the riddles of the
caskets. The first one, the gold one, states quite clearly "all that glitters is not
gold; often have you heard that told." Choosing the gold casket, thinking it holds
riches, reveals that appearances can be deceiving and that true value is not determined
by accepted standards of worth. Ultimately the casket that holds the key to Portia's
hand is made of lead, and Bassanio knows to choose this one.