Under what circumstances will light display total internal reflection?

light passing from a low-velocity medium to a high-velocity medium at an incident angle larger than the critical angle will be totally reflected

Explanation:

n=c/v

Index of refraction and velocity are inversely related. The higher the index, the slower the velocity. The law is that in order for there to be a critical angle or total internal reflection, the light must be traveling from a higher index of refraction medium towards a lower index of refraction medium. The incident angle has to be larger than the critical angle in order for it to be reflected, if they are equal than the angle of refraction creates a 90 degree angle, aka no critical angle.

a.   mass number from atomic number