When light travels from silicon (refractive index 3.48) toward oxide (refractive index 1.45), which is closest to the critical angle for total internal reflection (measured from the normal to the interface)?

Answer: B. About 25°

The critical angle θc is given by sinθc = n2 / n1; 1.45 ÷ 3.48 ≈ 0.417 gives θc ≈ 24.6°, or about 25°. Light striking the interface at angles greater than 25° from the normal does not exit into the oxide and is totally reflected. Total internal reflection occurs only when light travels from the higher-index side to the lower-index side, not in the reverse direction. In optical fiber, where the index contrast is small, the critical angle is large, 80° or more, so only light striking the boundary at shallow angles is confined. About 65° is the value obtained by mistakenly measuring the angle from the interface.