The object of cutting a diamond is to obtain an optimum balance of BRILLIANCE and FIRE. The cutter is obviously confined by optical laws.
Only one light ray is traced in this illustration, however, millions of light rays are taking as many different paths through the diamond at any one time.
When light strikes a diamond facet on the crown, approximately 17% of the light rays are reflected back, 83% of the rays enter the diamond.
Once inside the diamond, the light can exit only if it strikes the interface between diamond and air within the critical angle. If it strikes at a greater angle, all of the light is reflected back into the stone.
This is called Total Reflection.
In a well made diamond, a high percentage of light entering through the crown starts out by striking the pavilion outside the critical angle. Thus it is totally reflected to the other side of the pavilion. There it strikes outside the critical angle again, and again is totally reflected, this time towards the crown, where it strikes within the critical angle and leaves the diamond in the direction where it can be seen.
The CRITICAL ANGLE, ( indicated by the red cones) of Diamond is 24.5º to the normal (imaginary line perpendicular to the interface of diamond and air). Diamond, with it's small critical angle relative to other gems, creates a smaller window through which the light can escape. This is one reason why diamond is potentially more brilliant than most other gems.