Chamber 8 is where you first start facing a little bit of danger; as GLaDOS puts it:
Please note that we have added a consequence for failure. Any contact with the chamber floor will result in an unsatisfactory mark on your official testing record, followed by death. Good luck.
—GLaDOS
The first thing to do is to reorient the pellet on the left so that it's flying out of the orange portal. To do this, place a portal on the burn mark on the far wall where the pellet bounces. The pellet will fly through and come out of the fixed orange end of the portal. Now, before it bounces back through the portal towards the pellet launcher, re-place the blue end of the portal on top of the red area lit by the pellet receiver; thus when the pellet does bounce back into the orange end of the portal, it will come out the blue end heading straight towards the pellet receiver.
To get to the exit, you must get onto the platform, but this has to be a two-stage process, as the orange end of the portal is fixed. First, create a portal anywhere accessible from your current location, and go through it, bringing you to the ledge. Now, create a portal on the wall beside the near end of the platform's track, and wait until the platform is beside it before going through and riding to the chamberlock.
Very impressive. Please note that any appearance of danger is merely a device to enhance your testing experience.
—GLaDOS