Which term identifies the boundary condition at which total internal reflection begins?

• A. Angle of Incidence
• B. Angle of Refraction
• C. Normal
• D. Critical Angle

Answer: (D)

The boundary value where total internal reflection begins is the critical angle. This occurs when light travels from a denser medium to a less dense one. Using Snell’s law, n1 sin(theta1) = n2 sin(theta2), the threshold is reached when the refracted ray just skims the interface, so theta2 = 90 degrees. That gives sin(theta_c) = n2/n1, defining the critical angle theta_c. For incidence angles smaller than theta_c, refraction occurs with a refracted angle less than 90 degrees. For angles larger than theta_c, no refracted ray exists and all the light is reflected internally. The other terms are related to the geometry but do not specify this threshold: the angle of incidence is the incoming angle, the angle of refraction is the angle in the second medium, and the normal is the line perpendicular to the interface.