Deviation of Light by a Plane-Mirror. A ray of light that is incident on to the surface of a plane-mirror is reflected with the angle of incidence equal to the angle of reflection. ... The total angle between the straight-line path and the reflected ray is twice the angle of incidence.
as its a perpendicular relationship we have between angle of incidence and normal to the point of incidence , so, its twice ,
there are even mathematical proof , if u want then let me know .
HOPE IT CLEARS ..