Level 15 in “Portal” focuses on the preservation of momentum between portals to make long jumps. In a nutshell, you must fall a long distance from one portal placed high up directly into another portal placed on the ground below it. This will preserve the momentum built up from the fall and fling you out over long distances. You will need to use this trick many times in the level. You will also need to redirect energy pellets with portals to activate moving platforms.
Fire a blue portal onto the panel that protrudes from the wall overhead. Place an orange portal on the floor below it.
Jump through the orange portal, and as you fall from the blue portal aim so that you will fall through the orange portal again.
Walk to the next room and shoot an orange portal at the scorch mark left by the energy pellet. Place a blue portal roughly five tiles to the left so that when the energy pellet travels out of it, the pellet will go into the chamber that houses the pellet receptor that is sealed off by the incandescent particle field.
Wait for the energy pellet to leave the blue portal, and then step through the incandescent particle field yourself. Place a blue portal on the scorch mark left by the pellet on this side of the particle field, and an orange portal above the pellet receptor.
Wait for the pellet to travel through the portals and land in the receptor. Jump on the lift that lowers, and ride it up.
Walk into the next part of the chamber and look above the door you entered. Shoot a blue portal onto the panel that protrudes from the wall.
Walk up the stairs on the left wall and place an orange portal in the shallow pit.
Jump through the orange portal. As you fly out of the blue portal, shoot another orange portal at the point on the floor where it appears that you will land, and aim to fall through it.
Look up for the panel that protrudes from the wall in the next area. Place a blue portal on this and then shoot an orange portal onto the ground underneath it. Go through the orange portal and aim to fall through it again after you exit the blue portal.
Shoot a blue portal onto the left side wall with the energy pellet scorch mark on it. Place an orange portal on the right side wall directly opposite the blue portal. Wait for the energy pellet to go through both portals and then land in the energy pellet receptor.
Place a blue portal on the highest point of the wall that faces the incandescent particle field and then shoot an orange portal onto the floor beneath it. Go through the orange portal and then fall through it again as you have done before.
Stand on the edge of the corridor with the mobile platforms. Shoot a blue portal onto the far wall, opposite from where you stand, and then place an orange portal on the wall next to you.
Walk through the orange portal when you see a platform appear just outside of it. Turn left and shoot another blue portal to the end of the corridor. Ride back to the orange portal.
Go through the orange portal again when another platform appears outside of it. Turn right and shoot a blue portal onto the floor of the room you can see. Ride back to the orange portal — or create a new one in the wall in front of you — and go through it.
Ride a lift up to one of the side rooms. Place a blue portal in the side room you are in and shoot an orange portal over to the side room on the opposite side of the chamber.
Wait for the energy dispenser to create a fresh energy pellet, and then press the switch in the room you are in. Travel through the blue portal and press the switch in the opposite room.
Shoot a blue portal onto the panel that juts out in the main chamber, opposite the two doors you just opened with the switches. Shoot an orange portal onto the scorch mark where the energy pellet has impacted.
Get on the lift that lowers after the energy pellet has hit the energy pellet receptor, and then walk through the exit to complete the level.
Place the portals quickly after you have pressed the two switches in the last chamber, because the doors revealing the energy pellet receptor will close shortly after opening.
Traveling through an incandescent particle field will remove any portals you have placed.