A simple test. This will be test chamber 1 after you reacquire the dual portal device. GLaDOS will talk about how test subjects react to deadly lasers, you simply need to walk over to the platform near the door (if you don’t see it you can do the next step to see where it comes from) and then place one portal on the roof where the laser hits it, and another on the roof above the small dome-like laser receptor (look to where the dotted blue line ends). The platform will rise giving you access to the exit.
If you had to place the portals first to see where the platform is, simply move one and it will come back down so you can get on it before replacing the portal to go back up.