Q.What is angle of deviation due to reflection?

The two laws of reflection state that (a) the incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane, and (b) the angle of incidence $i$ equals the angle of reflection $r$, both measured from the normal. These laws hold at every point of any reflecting surface, plane or curved -- a flat mirror keeps parallel rays parallel after reflection (regular reflection), while a rough surface scatters them (irregular reflection), even though the law $i=r$ is obeyed locally at each point. For a plane mirror, tracing rays from a point object $A$ to the mirror and applying congruent triangles ($\angle AON=\angle DAO$, $\angle BON=\angle OA'D$) shows the image $A'$ forms exactly as far behind the mirror as the object is in front, so $AD=A'D$. This gives the standard characteristics of a plane-mirror image: virtual (cannot be caught on a screen), erect, laterally inverted, of the same size as the object, and located at the same perpendicular distance behind the mirror as the object is in front. A related result is that a person needs a mirror only half their own height to see their full reflection, independent of how far they stand from it. When an object sits between two plane mirrors inclined at angle $\theta$, the number of images formed is $n=(360/\theta)-1$ when $360/\theta$ is an even integer (any placement) or an odd integer (symmetric placement), and $n=360/\theta$ when $360/\theta$ is an odd integer but the object is placed unsymmetrically. A plane mirror usually forms a virtual image from divergent (real-object) rays, but can form a real image in front of it if it receives converging rays.