Critical angle Sin \(\theta\)\(_c\) = \(\frac{n_2}{n_1}\)

where n\(_1\) = refractive index of the medium ( 1.4 given )

n\(_2\) = refractive index of the second medium( air ≈ 1.0)

Sin\(\theta\)\(_c\) = \(\frac{1}{1.4}\) ≈ 0.7143

\(\theta\)\ = Sin\(^{-1}\)( 0.7143) ≈ 45.58º ≈ 46º

The minimum internal angle for total internal reflection to take place in the medium is approximately = 46º