The modern cut was designed in a way to contain the light travel path avoiding the pathway of escape within the critical angle 24.5 degrees pictured as a cone.

The crown angles on a modern cut were designed to allow the light to escape within the 24.5 degrees critical angle pictured as a cone.

The third aspect of light travel in a diamond deals with the internal containment (and distribution). This is the key component in designing the angles of a well cut diamond. It isn’t just an accident that light enters a diamond, refracts, reflects and disperses into a kaleidoscope of color. The containment of this light was critical in making diamond shine the way it does in the modern cut. If you understand the way light is reflected or is transmitted by a diamond, this part is easy. Inside the diamond there is a very specific path for the light to follow and escape forever. This escape route is the critical angle, which is 24.5 degrees. This is actually more accurately described as an imaginary cone where the point starts at the surface entry or exit point and flares outward at 24.5 degrees. After light enters the diamond, the bottom angles are designed in such a way that the light reflection creates an angle greater than 24.5 degrees and as a result the escape route is eliminated. As you can see in in the following illustrations, the final escape route is planned in order to allow the disperion and brilliance to be viewed from the crown facets. Just as in the stone that enters the water at too steep an angle, the light also will escape by contacting the surface at too steep an angle.