Chapter 3: Test Chamber 17

In here we have a big button that opens the exit door, which is up on a ledge that you can't reach right now. There is a weighted cube on a light bridge, but the cube is blocking a laser. There's a laser receptacle on the wall across from it. When you take the cube away, the laser goes into the receptacle, which makes a platform near the exit door go up.

First make a portal at the end of the light bridge and another portal that you can walk into. Walk along the bridge to reach the cube. There is a portal directly behind you that you just came out of. You want that portal to stay there.

Notice that on this same wall, there is another white spot that can take portals. If you make a portal on that wall for the light bridge to come out from, the light bridge will be going directly over the big red button. Do that.

Take the cube and go through the portal behind you. Put the cube on the light bridge such that it's blocking the laser and is directly above the big red button.

Jump down to the floor and stand on the moving platform near the exit door. Now create a portal of the color that the light bridge is coming out of. You can create it next to the portal that the light bridge is going into. This will make the cube fall onto the big red button while simultaneously unblocking the laser. This will cause the platform that you are standing on to rise up, and the exit door will be open.