How is the appearance vs. reality theme illustrated in act 1 scene 1 of Cordelia's dialogue with King Lear?

The dialogue between Lear and his three daughters in the opening scene of the play sets up what will follow.
The theme of appearance versus reality first becomes evident in the flattery of Goneril and Regan, who speak before Cordelia; their goal is to flatter Lear sufficiently that he will believe they love him most, but not everything they say is a lie. Indeed, Regan's comment that she is "made / of the self-same metal that my sister is" is completely true: the two older sisters are arguably as bad as each other. However, Lear is too swayed by their flattery to understand that the traits these sisters share are not positive ones: he takes the sisters' words as they appear, rather than perceiving the true reality of the situation.
Cordelia, however, is more perspicacious and has hope that her father will be more perceptive than he proves to be. She states to herself, "My love's more richer than my tongue," assuring herself that Lear will not be swayed by her sisters' false words. Unwilling to "heave [her] heart into [her] mouth," she declares that she has "nothing" to say, but Lear is unhappy with this. He does not appreciate the honesty in what Cordelia is saying and states, "Nothing will come of nothing: speak again." This motif of "nothing," and what it signifies, recurs throughout the play; these lines foreshadow Lear's Fool's comment later that the king is "an O without a figure. . . . I am a fool, thou art nothing."
Cordelia's speech to Lear is perfectly balanced: she says she will "return those duties back as are right fit" but that she "shall never marry like my sisters / To love my father all." When Lear describes her as "untender," she counters, "So young, my lord, and true." Again, in this comment, Cordelia speaks nothing but the truth.
Lear, unfortunately, misreads the situation entirely. He reads Goneril's and Regan's flattering words as evidence of their love, and he sees Cordelia's vow of honesty as disrespect. In fact, as will become clear, Cordelia alone is "true" to her father, and Lear's poor choice here will haunt him.

Cordelia's older sisters, King Lear's other two daughters, have just lied to the king in declaring their love for him. In fact, they do not care about him at all and simply want to get hold of their share of the kingdom. They will say anything to get the power they crave.
Cordelia, more perceptive than her father, realizes her sisters are laying the flattery on thick. She is so disgusted with their lying flattery that when her turn comes to praise her father, she refuses to imitate their insincerity. In fact, she loves her father too much to lie to him. However, when she refuses to flatter him, the king is so enraged he disinherits and banishes her.
Lear's tragedy is that he mistakes his older daughters' words for their hearts and cannot see the sincerity of his youngest daughter's heart beneath her words. He mistakes words (appearances) for reality.