The incidence angle at which the angle of refraction is 90 degrees is called the critical angle.

Here's why:

* Refraction: When light passes from one medium to another (like from air to water), it bends. This bending is called refraction.

* Critical Angle: The critical angle is the specific angle of incidence where the refracted ray travels along the boundary between the two mediums. At this angle, the angle of refraction is 90 degrees.

* Total Internal Reflection: If the angle of incidence is greater than the critical angle, the light will not be refracted but instead will be reflected back into the original medium. This is called total internal reflection.

Let me know if you'd like to know more about refraction, critical angles, or total internal reflection!