Light travels inside fresh water of index 1.333 and meets a layer of ice of index 1.309. What is the smallest angle at which it is totally internally reflected?

1. The light starts in the water, so n1 = 1.333, and the ice it reflects from gives n2 = 1.309. 2. Since 1.333 is greater than 1.309, the condition for total internal reflection is satisfied. 3. Substituting, theta_c = inverse sine of (1.309 divided by 1.333) = inverse sine of 0.982. 4. This gives theta_c = 79.1 degrees, so only rays beyond 79.1 degrees are totally reflected. 5. The angle is close to 90 degrees because the two indices are so nearly equal, leaving only a narrow band of grazing rays. 6. The 48.6 degrees option is the water to air value and ignores the ice entirely. _Source: OpenStax College Physics (CC BY 4.0), Ch 25 "Geometric Optics", section 25.4 Total Internal Reflection_