When the incident ray is at an angle of 90 degrees to the prism, it means the ray is hitting the prism perpendicular to its surface. Here's what happens:

* No Refraction: Since the angle of incidence is 90 degrees, the angle of refraction is also 0 degrees. This means the light ray doesn't bend as it enters the prism.

* Straight Path: The light ray continues to travel in a straight line through the prism.

* Possible Internal Reflection: If the angle of incidence inside the prism (relative to the other surface) is greater than the critical angle, the light will undergo total internal reflection. This depends on the prism's material and the angle at which the prism is cut.

In Summary:

A light ray hitting a prism perpendicularly (90 degrees) does not experience refraction, meaning it travels straight through. The only possibility for a change in direction is if the light undergoes total internal reflection inside the prism, which is dependent on the prism's material and shape.