Refractive index for medium M₁ is

$\mu$₁ = ${{3 \times {{10}^8}} \over {1.5 \times {{10}^8}}}$ = 2

Refractive index for medium M₂ is

$\mu$₂ = ${{3 \times {{10}^8}} \over {2.0 \times {{10}^8}}}$ = ${3 \over 2}$

For total internal reflection, sini $\ge$ sinC

But sinC = ${{{\mu _2}} \over {{\mu _1}}}$

sini $\ge$ ${{{\mu _2}} \over {{\mu _1}}}$ $\ge$ ${{3/2} \over 2}$

$\Rightarrow$ i $\ge$ sin^$-$1 $\left( {{3 \over 4}} \right)$