hen forced into a circular path, electrons traveling at nearly the speed of light emit bright x-ray and ultraviolet light that is directed down 27 different beamlines to experimental end stations at the Advanced Light Source. Why are light source experiments always covered in aluminum foil? To create the excellent vacuum required, researchers must "bake out" the light source beamlines to their experiments. The aluminum foil reflects the heat of the bake-out, to create a better vacuum.