Is 'iddah obligatory for a wife who was divorced due to her husband's infertility? And what is 'iddah, what is the reason for its obligation, and how long does it last?

The 'iddah (waiting period) is a period during which a woman waits to ascertain the clearness of her womb, or for devotion, or to mourn her husband. The 'iddah for a divorced woman who menstruates is three menstrual cycles.