Complete reflection of a ray of light in a medium such as water or glass, from the surrounding surfaces back into the medium. It occurs when the angle of incidence is greater than a certain limiting angle, called the critical angle. In general, it takes place at the boundary between two transparent media when a ray of light in a medium of higher index of refraction approaches another medium of lower index of refraction at more than the critical angle. At all angles less than the critical angle, both reflection and refraction occur. Total internal reflection is responsible for rainbows, atmospheric halos, the sparkle of a diamond, and the path of light through optical fibres.

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Any engine in which a fuel-air mixture is burned in the engine proper so that the hot gaseous products of combustion act directly on the surfaces of its moving parts, such as those of pistons (see piston and cylinder) or turbine rotor blades. Internal-combustion engines include gasoline engines, diesel engines, gas turbine engines, pure jet engines, and rocket engines and motors, and are one class of heat engines. They are commonly divided into continuous-combustion engines and intermittent-combustion engines. In the first type (e.g., jet engines) fuel and air flow steadily into the engine, where a stable flame is maintained for continuous combustion. In the second (e.g., gasoline–reciprocating-piston engines), discrete quantities of fuel and air are periodically ignited. Seealso automobile industry, machine, steam engine.

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Medical specialty dealing with the entire patient rather than a particular organ system, covering diagnosis and medical (rather than surgical) treatment in adults. Its development began in the 17th century with Thomas Sydenham's concept of disease, but until disease-specific therapies were developed in the 20th century, internists could do little to treat diseases. As more specific treatments became available, medical knowledge increased, and subspecialties in specific organ systems were defined, internal medicine became recognized as a specialty.

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See Internal