The computer warns you that the high energy pellet is dangerous, and if it hits you it could cause vaporisation. Notice that the pellet is bumping against an orange portal. What you need to do is make the pellet go into the particle containment thing on the floor, and all you need to do for that is create a blue portal on the ceiling where the containment thing is shining a light. The pellet will shoot into the containment thing, making the platform descend to ground level where you can reach it.
Notice by the way that you can't make portals on the walls in here. They are made of some sort of material that doesn't allow the creation of portals, or something.
The computer congratulates you, although the impersonal nature of the congratulation is apparent in the 'subject name here' and 'subject hometown here' that haven't been replaced with your actual name or hometown.
Anyway, step onto the platform to get to the elevator to the next chamber.