Why does Floria Tosca become jealous after seeing Cavaradossi’s painting in Act I?

In Giacomo Puccini’s opera Tosca, Floria Tosca becomes jealous after seeing the portrait Cavaradossi is painting in the church of Sant’Andrea della Valle because she recognizes the subject as the blonde, blue-eyed Marchesa Attavanti.1

Tosca, who is raven-haired, is already suspicious of Cavaradossi’s behavior and his delay in answering the church door. Upon seeing the painting, she identifies the woman as the Marchesa, who has been praying in the church daily, and accuses Cavaradossi of infidelity. Although Cavaradossi tenderly reassures her of his fidelity and she is eventually convinced, her insecurity leads her to demand that he change the eye color in the painting to match her own.1