Total internal reflect occurs:

(A) when moving from a less optically dense (small n) medium to a more optically dense one, for rays near the surface interface
(B) when moving from an optically dense (large n) medium to a less optically dense one, for rays near the surface normal
(C) when moving from an optically dense (large n) medium to a less optically dense one, for rays near the surface interface
(D) when moving from a less optically dense (small n) medium to a more optically dense one, for all angles
(E) when moving from a less optically dense (small n) medium to a more optically dense one, for rays near the surface normal
(F) when moving from an optically dense (large n) medium to a less optically dense one, for all angles
(G) In all situations in which a light ray travels from one medium to another

Total internal reflection (TIR) is the phenomena for which the incident rays above a critical angle reflects back from the interface between two media. In this case no transmittance from one medium to the other occurs.

The key idea behind TIR lies with the Snell's Law:

[tex]n_1\sin(\theta_1) = n_2\sin(\theta_2)[/tex]

where n is the index of refraction and θ is the angle of incidence.

From this law, the critical angle for total internal reflection can be deduced:

[tex]\theta_c = \arcsin(\frac{n_2}{n_1})[/tex]

In order the arcsin term gives an angle θ, n2 must be less than or equal to n1, otherwise total internal reflection does not occur. Furthermore, the incident angle with respect to the surface normal, must be higher than this critical angle.

Hence, the incident ray should be close to the surface interface and move from an optically dense medium to a less dense one.