Consider the ray diagram for the refraction given below. The maximum value of angle 6 for which the light suffers total internal reflection at the vertical surface, is

For total internal reflection to occur at the vertical surface, the minimum angle the ray incident on vertical surface can make is
sin(11.25)=sin(45)
Hence, the maximum angle that the refracted ray at the horizontal surface makes with the vertical=90°-sin(45)
Hence, from Snell's law for reflection at horizontal surface,
sinθmax=1.25×sin(90°-sin(45))=1.25×cos(sin(45))=34
⇒θmax=sin(34)