State what would happen when an incident ray is equal to 90 degrees. 

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If the incident ray is equal to 90 degrees, nothing would reflect off of the mirror. That would be the result because the incident ray is travelling parallel to the surface of the mirror.  

The angle of incidence is measured from the normal, and the normal extends perpendicular from the surface of the mirror.  The angle of incidence is measured from that line.  

I believe that the question might be incorrectly assuming that the normal is 90 degrees instead of 0 degrees.  A line drawn straight up from the surface of the mirror could cause people to assume that the line represents 90 degrees.  Mirror rules are different.  The mirror itself is 90 degrees from the normal.  

The resulting reflection from an incoming ray of light can be explained using the law of reflection.  The law of reflection states that when a ray of light reflects off of a surface, the angle of incidence is equal to the angle of reflection.  For example, if the incident ray is at an angle of 37 degrees, then the reflected ray also has an angle of 37 degrees.  An incident ray of 0 degrees would reflect straight back at the same 0 degrees.  90 degrees wouldn't interact with the mirror at all.  

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