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      Medium X       |     Liquid (n = 1.29)     |     Medium Y
       (n = 1.54)          |                           |   (n = 1.44)
       ---------------------|---------------------------|------------------
            \   θ₁          |      θ₂ (critical angle)  |   No light enters
                  \              |      /                    |   here if TIR occurs
                         \             |     /                     
                                 \__________> | <-- TIR happens here if θ₂ ≥ critical angle