When the incident angle is equal to the critical angle, the angle of refraction is 90 degrees.

Here's why:

* Critical Angle: The critical angle is the angle of incidence at which the refracted ray travels along the boundary between the two mediums.

* Total Internal Reflection: When the angle of incidence exceeds the critical angle, the light is completely reflected back into the original medium. This is called total internal reflection.

Therefore, at the critical angle, the refracted ray is just grazing the boundary, making a 90-degree angle with the normal.