Light travels in two media M₁ and M₂ with speeds 1.5*10⁸ ms and 2.0*10⁸ ms⁻¹ respectively. the crtical angle between them is?
a. tan⁻¹(3/√7)
b. tan⁻¹(2/3)
c. cos⁻¹(3/4)
d. sin⁻¹(2/3