critical angle

critical angle (plural critical angles)

1. (physics) The smallest angle of incidence for which light is totally reflected from a boundary between one medium and a less refractive one
2. (aerodynamics) The smallest angle of attack of an airfoil at which lift suddenly decreases and drag suddenly increases
3. (architecture) The smallest angle of a ramp or stairs considered uncomfortable or unsafe

• Snell's law
• total internal reflection