I have a 2011 iMac with one Thunderbolt 1 port and one FireWire port. Is there a way to use one of those ports to connect the iMac to an HDMI laptop so that it can be used as an external display? Unfortunately this model of iMac does not have Mini DisplayPort.
Mactracker (backed up by Apple's specs here) indicates that the Thunderbolt port is also a video output:
- Thunderbolt: 1 - Thunderbolt 1 (up to 10 Gbps)
- Display: Thunderbolt digital video output (Support for up to two Thunderbolt displays)
You should be able to use a MDP to HDMI adapter like this one from Belkin.
In our testing at my workplace, the Belkin adapters are the most robust.
Apple's Target Display Mode page reads as if iMacs from 2009-2014 can be used as external displays for laptops, but not the other way around.