Question
Assertion: Critical angle is minimum for violet colour.
Reason : Because critical angle ${\theta _c} = {\sin ^{ - 1}}\,\left( {\frac{1}{\mu }} \right)$ and $\mu \, \propto \,\frac{1}{\lambda }$.
✓
Answer
For total internal reflection to just take place, $\mu \sin \theta_c=1$
$\Rightarrow \theta_c=\sin ^{-1}\left(\frac{1}{\mu}\right)$
As a fact, the refractive index for a ray is inversely proportional to its wavelength. This is the reason for deviation of red light to be least in a prism.
Thus $\theta_c$ decreases with $\lambda$
Thus violet color has the least critical angle.
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