critical angle

[ krit-i-kuhl ang-guhl ]

noun

1. Optics. the minimum angle of incidence beyond which total internal reflection occurs for light traveling from a medium of higher to one of lower index of refraction; the angle of incidence for which refracted rays emerge tangent to the surface separating two media, the light traveling from the medium of higher to the medium of lower index of refraction.
2. Also called angle of stall,. Aeronautics. the angle of attack, greater than or equal to the angle of attack for maximum lift, at which there is a sudden change in the airflow around an airfoil with a subsequent decrease in lift and increase in drag.

critical angle

noun

1. the smallest possible angle of incidence for which light rays are totally reflected at an interface between substances of different refractive index
2. another name for stalling angle

critical angle

/ krĭt′ĭ-kəl /

1. The smallest angle of incidence at which radiation, such as light, is completely reflected from the boundary between two media. At angles smaller than the critical angle, some of the radiation enters the second material and is refracted.

Word History and Origins

Origin of critical angle¹

First recorded in 1870–75

Example Sentences

Subjects were instructed to tilt the tube towards Kevin’s image using specific keys on a keyboard until they felt the tube had reached the critical angle at which it would tip over.

From Scientific American

As the airplane approaches that critical angle, the first event is a stall warning in the cockpit.

From New York Times

Officials also said Wassef contributed to the crash by allowing the plane to exceed the “critical angle of attack” during an emergency return to the airport.

From Seattle Times

Lien, the first set designer ever to be named a MacArthur “genius” fellow, ingeniously arranges her scenic design at a critical angle.

From Los Angeles Times

If one gently dribbles sand onto a flat surface, it will pile up until its slopes reach a critical angle.

From Scientific American