If the angle of reflection is 90 degrees for a light ray, what is its angle of incidence?

According to the law of reflection, angle of incidence and angle of reflection are equal. Angle of incidence is the angle an incident light ray makes with a normal drawn to the surface. Similarly, angle of reflection is the angle made by the reflected ray with a normal drawn to the reflecting surface.

In other words, angle of incidence = angle of reflection

Since, the angle of reflection in the given case is 90 degrees,...

According to the law of reflection, angle of incidence and angle of reflection are equal. Angle of incidence is the angle an incident light ray makes with a normal drawn to the surface. Similarly, angle of reflection is the angle made by the reflected ray with a normal drawn to the reflecting surface.

In other words, angle of incidence = angle of reflection

Since, the angle of reflection in the given case is 90 degrees, the angle of incidence can be calculated as:

angle of incidence = angle of reflection = 90 degrees.

Thus, an angle of reflection of 90 degrees is caused by an incident ray that has an angle of incidence of 90 degrees.

An angle of reflection of 90 degrees means that the reflected ray travels along the surface. This is because the incident ray is also traveling along the reflecting surface. 

Hope this helps.