Light travels inside fresh water of index 1.333 and meets the flat surface with air above, index 1.00. What is the critical angle?

1. The light starts in the denser medium, so n1 = 1.333 for the water and n2 = 1.00 for the air. 2. The critical angle formula is theta_c = inverse sine of (n2 divided by n1). 3. Substituting, theta_c = inverse sine of (1.00 divided by 1.333) = inverse sine of 0.750. 4. This gives theta_c = 48.6 degrees for a ray going from water to air. 5. Beyond that angle a swimmer looking up sees the underside of the surface act as a mirror. _Source: OpenStax College Physics (CC BY 4.0), Ch 25 "Geometric Optics", section 25.4 Total Internal Reflection_