There are 2 options to connect MacBook Air to your iMac.
Firstly by using mini displayPort cable and connect the 2 mac’s. Just display and audio will be transferred via the cable.
Secondly, use share screen option (this will be bit slow since all communication of data will be done via LAN)
To do this:
1. On MacBook Air
- Go to System Preferences > Sharing
- Tick “Screen Sharing”
- Allow access for “only these users”. Choose a few.
- Set the “Computer Name”
2. On iMac
- Open spotlight. Type “Screen Sharing”
- Type the computer name you want to connect to
- Or go to Finder and double click on the computer under “Shared” category (sidebar)