Avraham and Sara both laughed when informed of the future birth of their son. God’s response to Avraham is non-judgmental. In contrast, He reprimands Sara for her laughter. Why did God respond to them both so differently?
There are those who claim that there is no significant difference between Sara and Avraham's laughter, and that Sara's admonition was directed to Avraham as well. However, Rashi explains that there is a difference between Avraham's laughter and Sara's laughter. Avraham believed the news of Yitzchak's birth and was pleased, but Sara did not believe it and made it into a mockery. Therefore, God rebuked Sara and not Avraham.
Rashi's interpretation may be based on the different descriptions of laughter. In the case of Avraham the Torah relates, “And Abraham laughed in his heart.” In contrast in the case of Sara it says "And Sarah laughed within herself." Avraham's laughter expressed legitimate astonishment, as the human mind may have difficulty understanding the possibility of the birth of a son to elderly parents.
However, Sara's laughter was "within herself" (בקרבה). This is the only instance in the Torah in which this word relates to a person's thoughts. Generally, the word בקרב, within, expresses a physical reality within people or places, such as "And the children struggled within her (בקרבה)" regarding Rivka's pregnancy carrying twins, or "For already two years of famine [have passed] within the land (בקרב הארץ)".
While Avraham's laugher expressed an understandable astonishment, Sara was reprimanded for her laughter, which was an inner expression of disbelief at the news of Yitzchak's birth.