The important thing to note about these natural light bridges is that they can go through portals, as you might expect.

Shoot a portal onto the wall that the bridge is touching. Then shoot a portal such that you can reach the small red button on the other side of the room by walking along the bridge. Press the small red button. Cubes will keep falling out of the cube chute over and over as each one falls into the toxic liquid and is destroyed.

Go back to the other side of the room where the bridge is shooting into the portal. Notice the portion of the wall to the left that sticks out a little. Put a portal into that wall for the light bridge to come out of. Then step through the portal that the bridge is shooting into. You will be standing over the pool of toxic liquid, so be really careful when making new portals.

Walk all the way to the end of the bridge to reach a set of narrow passages. Shoot a portal onto the wall to the left such that the bridge appears below you. You will land on it. Walk to the end of the bridge again and create a portal on the lower wall to the side. The bridge will appear below you once more.

The falling cube will either land on the bridge or hit the side and fall off. Even if it is falling, just walk over and catch it as it falls. Then go through the portal that the bridge is coming out of.

There is a big red button on the side of the room. Shoot a portal onto the wall such that it goes into the gap to the right of the glass that blocks the red button. Walk through the portal along the bridge and put the cube onto the red button.

The bridge that you created is too high up for you to reach, so make a bridge on a wall above the toxic liquid so you can jump down onto it. Then all you have to do is walk through the portal to get back to the ledge, then create another portal to reach the area with the small red button and the exit. Go to the elevator.