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(1) Correctly cut gem: Now, watch the light path. Note the rainbow of colored light, a consequence of dispersion .
(2) Gem is cut too shallow: Note how light leaks from the bottom of the gem . The gem would appear dark and dull.
(3) Gem is cut too deep: Again, note that light passes inside the critical angle at certain bottom facets, resulting in leakage from the bottom of the gem.
(i) Too shallow
(ii) Too deep
(iii) Just right.
DIAGRAM If the cut is too shallow or too deep, the light leaks from the pavillion. Note that the critical angle makes cones, through which light must pass inside of to escape