The science of physics can be divided into three distinct categories: classic, modern, and applied. Classic physics was first defined with the emergence of science during the Renaissance, and is often associated with the works of Newton and Galileo. Branches within classic physics includes mechanics, optics, acoustics, thermodynamics, and electromagnetism.
Mechanics is the study of the motion of objects. Optics is the study of how light behaves and interacts with matter, and is typically confined to the study of visible, infrared, and ultraviolet light. Acoustics is the study of sound, which are vibrations in various states of matter. Thermodynamics is the study of heat, the movement of energy within a system. Electromagnetism is the study of the behavior of electromagnetic fields, which includes both electrical and magnetic phenomena.
Modern physics emerged early in the 20th century after Einstein developed his theory of relativity. Much of modern physics builds on classic physics, but incorporates relativity or quantum theory. Branches within modern physics includes relativistic mechanics, quantum mechanics, quantum thermodynamics, and quantum electrodynamics (also known as QED).
Relativistic mechanics is the study of motion with regard to relativity and special relativity. Classic mechanics fits into this framework, but does not apply to motion at speeds approaching the speed of light. Quantum mechanics is the study of the behavior of matter on the atomic and quantum (particle) scales. Quantum thermodynamics is the study of energy on the atomic or quantum scale. Classic thermodynamics is an emergent phenomena of quantum thermodynamics. Quantum electrodynamics is the study of electromagnetism at the atomic and quantum scales; it is closely related to quantum mechanics.
Applied physics by far has the greatest number of branches. This is because applied physics is very specific. The best way to remember the branches of applied physics is to think of them as occurring on a scale from big to small.
On the big end of the scale, cosmology is the study of the Universe itself, its size, behavior, origin, and future. Astrophysics (which can be thought of as including cosmology), is the study of galaxies, stars, and other structures within the Universe. Planetary science is the study of planets, both in our solar system and in other star systems. Geophysics, which is closely related to planetary science, is the study of the physics of the Earth. This includes seismology, climatology, meteorology, geology, aeronomy (study of the structure of the atmosphere), glaciology, oceanography, hydrology, volcanology, geodesy (study of Earth's gravity) and terrestrial magnetism.
In the middle, instrumentation is the study and construction of measurement devices. Electronics is the study of electrons and electricity to produce electronic devices. Photonics, similar to electronics but related to optics, is the study of photons and light to produce photonic devices (like lasers). Fluid mechanics is the study of the behavior of fluids, both as gases and liquids. Plasma physics is the study of ionized gases (usually super hot). Condensed matter physics is the study of matter in the solid and liquid phases, as well as exotic phases like Bose-Einstein condensates and superfluids which occur at extremely low temperatures. This is the biggest branch of physics studied in the US; by some estimates, a third of US physicists study condensed matter physics. Condensed matter physics is closely related to chemistry, nanotechnology, and engineering. Thermal physics is the study of the applications of heat and energy at the macroscopic and microscopic scale.
At the small end of the scale, molecular physics is the study of the behavior and structure of molecules, which is closely related to chemistry. Atomic physics is the study of the structure and behavior of the atom; this is also closely related to chemistry. Nuclear physics is specifically the study of the nucleus of the atom, how protons and neutrons interact in various combinations. Finally, particle physics is the study of the behavior of quantum particles, including quarks, electrons, photons, and more exotic particles usually seen experimentally.
