Which term is defined as the angle of incidence at which the refracted ray is at 90 degrees?

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

Answer: (C)

The critical angle is the angle of incidence in the denser medium at which the refracted ray travels along the boundary, i.e., at 90 degrees to the normal. This occurs when light moves from a medium with a higher refractive index to one with a lower refractive index. Using Snell’s law, n1 sin(theta1) = n2 sin(theta2), set theta2 to 90° so sin(theta2) = 1. Then sin(theta1) = n2/n1, which defines the critical angle. If the incident angle is larger than this, refraction can’t occur and total internal reflection takes place.

The other terms don’t capture this boundary condition: the angle of incidence is just how the incoming ray meets the interface; the normal is the perpendicular line to the interface; the angle of reflection concerns the reflected ray, not the refracted one.