Step through and you'll fall onto the light bridge, note what colour of portal you used as your exit portal (the one leading to the light bridge) and reset it at the wall directly where the light bridge stops.
This will take you back to the start of the room. Halfway across this central walkway look to the wall on your immediate left. You should spot a wall panel made up of really small squares, fire your entrance portal nearer the top of it.
Turn around and edge yourself over the bridge (to your left) and fire your exit portal so a bridge appears almost directly below you.