Critical Angle

Critical Angle The angle of incidence of light measured from the normal (90%) beyond which Total Internal Reflection will occur. From diamond to air, this is a very low figure of 24.4°.
Crown The top part of a diamond, above the girdle.

Critical Angle
Greatest angle measured from normal at which light can be refracted out of a stone. Small angle at which light is totally internally reflected.
Crown
The part of a facetted gemstone above the girdle.

Critical Angle - The largest angle measured from the normal at which light can escape from and optically dense substance, and the smallest angle to the normal at which light is totally reflected within the dense substance.

critical angles The internal critical angle is the escape route of light. In order to capture light, the reflected light angle must be greater than 24.5 degrees. The pavilion's critical angle is 40.

Critical Angle
Critical angle is the largest angle measured from the normal angle from which light can escape, is an optically dense substance, ...

The Critical Angle: Each gem species, depending (with an inverse relationship) on its refractive index, has a pavilion faceting angle below which it loses brilliance. ...

Critical Angle...The angle at which a ray of light passes from one medium to another, as a gemstone and air.

The critical angle in Figs. 187 a and b is shown by a cone; by this means the critical angle surrounding the incidence normal on the pavilion facets can be envisaged.

The "critical angle" at which a light beam intersects with a given series of facets is the angle at which total "internal reflection" is achieved.

For diamond this critical angle, as it is called, is very nearly 24deg from a perpendicular to the surface.

One test to determine a gem's identity is to measure the refraction of light in the gem. Every material has a critical angle, at which point light is reflected back internally.

If the facets are cut below the critical angle for the particular material, light will pass right through the stone instead of being reflected back towards your eye. When this happens the gem will lack sparkle and brilliance.

See also: Crystal, Stone, Rough, Color, Gemstone