Is it possible to use an iMac's screen as a second monitor?
I don't know your iMac spec, but look at Apple knowledge base


  With Target Display mode, you can use your iMac (27-inch, Mid 2010) or
  iMac (27-inch, Late 2009) with Mac OS X as an external display.
  Connect any computer or other device with a Mini DisplayPort to your
  27-inch iMac using a Mini DisplayPort to Mini DisplayPort cable or
  using a converter that converts other electrical, video, and audio
  protocols from another source device to Mini DisplayPort compliant
  signals.  (Cables and converters available separately.)