Answer :
Final answer:
The critical angle of an air-glass boundary, such as 39.3 degrees, represents the angle of incidence where total internal reflection occurs when light passes from a denser medium to a less dense one. The correct option is a.
Explanation:
The critical angle of an air-glass boundary, such as the 39.3 degrees mentioned, best describes the angle of incidence above which total internal reflection occurs. This occurs when light is trying to move from a medium with a higher refractive index (glass in this case) to one with a lower refractive index (air), and the angle of incidence is sufficiently large such that the refracted ray would have to travel along the boundary. This cannot happen, so instead of refracting into the air, the light is entirely reflected back into the glass. According to Snell's law, the critical angle (°c) can be calculated using the equation °c = arcsin(n2/n1), where n1 is the refractive index of the first medium (glass) and n2 is that of the second medium (air), and n2/n1 is less than 1. This angle is specific to the pair of materials involved. The 39.3 degrees mentioned is likely a rounded value or specific to a certain type of glass with a refractive index slightly different from other glasses for which you might see critical angles like 41.8 degrees or similar values cited.