The true statement about boundary refraction and critical angles:

• A. Refraction occurs with any angle; no total internal reflection.
• B. Snell's law does not apply to this boundary.
• C. The refracted angle is always equal to the incident angle.
• D. If the incident angle exceeds the critical angle, total internal reflection occurs.

Answer: (D)

When light moves from one medium to another, its path bends according to Snell's law, which relates the incident and refracted angles to the refractive indices. A critical angle exists when light goes from a denser medium to a less dense one. For incident angles smaller than this critical angle, the light refracts into the second medium at an angle less than 90 degrees. At the critical angle itself, the refracted ray skims along the boundary (90 degrees to the normal). If the incident angle is larger than the critical angle, there is no refracted ray in the second medium because Snell's law would require a sine greater than 1. Instead, all the light is reflected back into the first medium—this is total internal reflection.

This is why the statement about total internal reflection occurring when the incident angle exceeds the critical angle is the true one. The other ideas don’t hold: refraction does not occur for all angles once you’re past the critical angle; Snell's law still governs the boundary (its requirement would be impossible to satisfy for a refracted ray beyond the critical angle); and the refracted angle is not generally equal to the incident angle.