Go down to the floor and grab a laser cube. Then look at where the laser is shining, next to a wall. Create a portal in the area that the laser is shining at, and create a portal of the other color nearby, then take the laser cube through. There is a laser relay to the left of the laser emitter. Use the laser cube to aim the laser at the relay. Then go to the edge of this area and drop down to the floor.
Look next to the area where you first entered the chamber. There is another laser cube there. Put a portal on the ceiling above it, then a portal of the other color someplace that you can reach. Go through. You'll fall onto the platform with the laser cube. Pick up the cube and take it down to the main area.
Across from the area where you first entered the chamber, there is a portal wall with a green target painted on it. It's marked by a couple of signs. Put a portal on the wall with the target on it. Then create a portal on the floor near you. Pick up the laser cube and drop it through the portal. It will land on a catapult and will be shot straight up into the air. Before it falls through the portal, create a portal on the wall high up on the ceiling that is across from the laser relay that you lit up. If you did it correctly, the laser cube will be shot through the portal and will land in the area where the laser is currently shining.
Leave the floor portal where it is and create a portal of the other color on the ceiling with the green target. Fall through the floor portal, then once again create a portal on the high ceiling so you are shot to the area where you sent the laser cube a moment ago.
Move the laser cube around until you can set it down such that the laser shines through both the laser relay near you and the laser relay near the chamber number sign.
Drop down to the floor once more. There is a third laser cube on a platform high up near one corner of the room. Create a portal on the green target wall, then one on the floor near you. Drop in. While you're falling, create a portal on the wall facing the laser cube near the corner. With luck, you'll knock the laser cube off of the ledge and onto the floor some place where you can reach it.
The laser should be shining into the area with the door that you entered the chamber through. This area has a white ceiling that takes portals. Create a portal on the ceiling there, then a portal of the other color somewhere near you. Go get the laser cube that you knocked off of the ledge and go through the nearby portal to land where the laser is shining. Use the laser cube to shine the laser at the four turrets by the exit. Then shine the laser into the laser receiver to the left of where the turrets were. This makes a platform come up under the laser receiver.
Drop down to the floor. Look at the wall opposite the platform that just came up. There is a white wall there that you want to come through to reach that platform that you just raised. But first, create a portal on the wall with the green target on it, then create another on the floor near you. Drop in. While you are falling, create a portal on that wall that is across from the platform that you raised. You will land on it and can jump over the laser to reach the exit.