01	Acoustics	Generation and propagation of acoustic waves, including ultrasonic and infrasonic radiation; Underwater acoustics. For noise pollution, See 24/02, Noise Pollution and Control. For acoustooptic and optoacoustic devices, See 09/06, Acoustooptic and Optoacoustic Devices. See also 17/01, Acoustic Detection and Detectors.
02	Crystallography	Study of crystal structure; Theory, techniques and mechanics of crystal growth. Includes grain structures and crystal lattice defects. For the crystal structure of metals, See 11/06/01, Properties of Metals and Alloys. For liquid crystals, See 07/04, Physical Chemistry.
03	Electricity and Magnetism	Theoretical and experimental studies of electrical and magnetic phenomena; Circuit theory. For geomagnetic fields, See 08/04, Geomagnetism. For radiofrequency wave propagation, See 20/14, Radiofrequency Wave Propagation. For electromagnetic pulses, See 20/15, Electromagnetic Pulses.
04	Fluid Mechanics	Theoretical and experimental studies of the dynamics and statics of fluids, including aerodynamics and hydrodynamics. For all aircraft applications, See 01/01, Aerodynamics. For missile applications, See 16/02/01, Guided Missile Dynamics, Configurations and Control Surfaces. For spacecraft applications, See 22/03, Spacecraft Trajectories and Reentry. See also 09/02, Fluidics and Fluerics, 20/09, Plasma Physics and Magnetohydrodynamics, and 13/07, Hydraulic and Pneumatic Equipment.
05	Atomic and Molecular Physics and Spectroscopy	Study of the structure of atoms and molecules and of the interactions between atoms and molecules; Processes which involve the interaction of radiant energy with matter; Infrared, optical and ultraviolet spectroscopy; Atomic and molecular spectroscopy; Electron paramagnetic resonance; Nuclear magnetic resonance; Quantum mechanics used to determine the orbitals, energy levels or properties of atoms and molecules. For interactions involving elementary particles and Nuclear energy levels, See 20/08, Nuclear Physics and Elementary Particle Physics. For the identification of inorganic and organic chemicals by means of their spectra, See 07/02, Inorganic Chemistry, and 07/03, Organic Chemistry. For astronomical spectroscopy, See 03/02, Astrophysics.
06	Optics	Generation, propagation and interaction with matter of electromagnetic waves in the infrared, visible and ultraviolet regions of the spectrum; Theory and design of optical equipment. For optical equipment used for specific applications, see group where application is treated. For identification of inorganic and organic chemicals by means of their spectra, See 07/02, Inorganic Chemistry, and 07/03, Organic Chemistry. For spectroscopy applied to atomic and molecular structures, See 20/05, Atomic and Molecular Physics and Spectroscopy. For Astronomical spectroscopy, See 03/02, Astrophysics. For electron optics, See 20/08, Nuclear Physics and Elementary Particle Physics. For microwave optics, See 20/14, Radiofrequency Wave Propagation. For fiber optics, See 20/06/01, Fiber Optics and Integrated Optics. See also 17/05, Optical Detection Detectors; 17/05/01, Infrared Detection and Detectors, and 17/05/02, Ultraviolet Detection and Detectors.
06/01	Fiber Optics and Integrated Optics	Theoretical and technological studies of the transmission of light through fibers of glass, plastic and other transparent materials; Studies of integrated optical circuits. For the application of fiber optics or integrated optics, see the group where the application is treated.
07	Particle Accelerators	Design and operation of particle accelerators; Betatrons; Cyclotrons; Synchrotrons. Includes accelerator target design and accelerator particle beam control. For directed energy weapons, See 19/12, Directed Energy Weapons.
08	Nuclear Physics and Elementary Particle Physics	Properties and reactions of elementary particles, gamma rays and x rays; Nuclear reactions, nuclear properties and nuclear structure; Electron optics. For atomic and molecular structure and spectra, See 20/05, Atomic and Molecular Physics and Spectroscopy. For nuclear magnetic resonance, See 20/05, Atomic and Molecular Physics and Spectroscopy. For solar and stellar nuclear physics, See 03/02, Astrophysics.
09	Plasma Physics and Magneto-hydrodynamics	Properties and actions of plasmas, including pinch effect, plasma ocillations, plasma jets. For magnetohydrodynamic(NHD) generators, See 10/02, Electric Power Production and Distribution. For plasma propulsion systems, See 21/03, Electric and Ion Propulsion. For astrophysics, See 03/02, Astrophysics. For aeronomy, See 04/01, Atmospheric Physics.
10	Quantum Theory and Relativity	Relativistic and nonrelativistic quantum theory; Relativity theory; Quantum mechanics and quantum statistics. For quantum chemistry, See 07/04, Physical Chemistry.
11	Mechanics	Dynamics and statics of solid bodies; Structural mechanics; Kinetics; Kinematics; Equilibria; Stress Analysis; Buckling; Plasticity; Mechanical shock and vibration; Theory of structural shells.
12	Solid State Physics	Properties exhibited by atoms and molecules because Of their association and regular periodic arrangement in crystals; Properties of solids at cryogenic temperatures; Fundamental research and theoretical studies of semiconductors; Band structure of solids. For solid state electronic devices, See 09/01, Electrical and Electronic Equipment. For the structure of crystals, See 20/02, Crystallography. For the structure and property of metals, See 11/06/01, Properties of Metals and Alloys.
13	Thermodynamics	Thermodynamic theory; equations of state; Free energy; Enthalpy; Entropy; Thermodynamic cycles; Theoretical studies of heat transfer; Cryogenic phenomena and methods; Kinetic theory of gases. For properties of solids at cryogenic temperatures, See 20/12, Solid State Physics. For thermal radiation properties such as absorptance, emittance, reflectance, and transmittance, See 20/06, Optics.
14	Radiofrequency Wave Propagation	Generation and propagation of radiofrequency waves and radar signals. Includes microwave optics.
15	Electromagnetic Pulses	Studies of the properties and effects of electromagnetic pulses from all sources; Studies of the generation of electromagnetic pulses by non-nuclear sources. For the generation of electromagnetic pulses by nuclear explosions, See 19/11, Nuclear Weapons.