Light travels in two media \( M_1 \) and \( M_2 \) with speeds \( 1.5 \times 10^8 \, \text{ms}^{-1} \) and \( 2.0 \times 10^8 \, \text{ms}^{-1} \) respectively. The critical angle between them is:
\( \tan^{-1}\!\left(\dfrac{3}{\sqrt{7}}\right) \)
\( \tan^{-1}\!\left(\dfrac{2}{3}\right) \)
\( \cos^{-1}\!\left(\dfrac{3}{4}\right) \)
\( \sin^{-1}\!\left(\dfrac{2}{3}\right) \)