Thank you in advance! Explain the formulas used please!

A light ray enters the end of a plastic light pipe (with refractive index n2 = 1.55) that is surrounded by water (with refractive index n1 = 1.33). The ray is refracted upon entering one end of the light pipe and then reflected off the inside, as illustrated below: n1 n2 02 031 In degrees, what is the largest angle of incidence 0, that will result in total internal reflection of the ray? Hint: work backwards from 03 to 02 to 01