The Preliminary Exam is a comprehensive two-day exam covering topics in all areas of physics. It is offered twice per year and passing it is a requirement for the physics PhD program.
Consistent pressure GradOPS has led to substantial revisions of the preliminary exam in recent years. The exam is now shorter and focuses on topics covered in a typical undergraduate physics curriculum, rather than on material covered in a first year graduate program.
GradOPS maintains a library of prelims from previous years, along with solutions. These prelims are available on this Google drive. Keep in mind that many of the older prelims will not be very representative of what to expect, for the reasons discussed above.