Physics at its most basic definition is the study of matter and energy. It is the widest ranging of the classical natural sciences in that it crosses into the studies of chemistry, biology, and astronomy, and it also is heavily intertwined with the study of mathematics. Physics can be divided into two broad categories, classical and modern. Classical physics is really pre-20th century physics. It includes classical mechanics (laws of motion), thermodynamics (heat), magnetism, and wave mechanics (sound and light). Modern physics is more recent with the study of particle physics and quantum mechanics. Modern physics is used to describe phenomena that were not readily observable until more modern instrumentation was available.