Can an iMac be used as a dual monitor for the powerbook laptop?

Yes, an iMac can be used as a dual monitor for a MacBook or PowerBook through a feature called Target Display Mode, which is available on certain older iMac models. To use this feature, the iMac must be running macOS High Sierra or earlier and have the appropriate Thunderbolt or Mini DisplayPort connections. However, more recent iMacs do not support this feature, so it's essential to check the specific model compatibility. Alternatively, third-party software solutions can allow screen sharing or extended display functionality.