The original iMac was not designed to support an external display. The iMac (Slot Loading) DV models, and those introduced after it, do support video mirroring through the built-in VGA port. Refer to your owners documentation to determine if your iMac model supports an external display.
Refer to article 58467: "
iMac (Slot Loading): Connecting External Display to VGA Port" for additional information.
Apple designed the iMac with a very sharp, 15-inch internal display but did not include a video-out port for connecting a second display. iMac is designed for the consumer market, and one of the themes incorporated in iMac was simplicity.
Note: The information in this article also appears in the following Knowledge Base articles: