Critical angle will be minimum for violet colour of the spectrum.
We know that refractive index μ is given as c/v where c is the velocity of light and v is the velocity of the respective colour in the medium.
In refraction , the velocity of the red colour is maximum and that of the violet colour is minimum.
Also we know that :
μ = 1/sin C where C is the critical angle.
Substituting μ= c/v , we get,
c/v = 1/sin C
hence v ∝ sin C
as a result C will be minimum for minimum v and that is violet colour