When we stand in front of a flat mirror, we have the impression that our image is formed in the mirror itself. But what really happens is that our image is formed behind the mirror, that is, there is the formation of a virtual image.
In this way, we can define a flat mirror as being a very well-polished metallic surface. Generally, a glass plate is used where a very thin layer of silver or aluminum is deposited on one side.
In the figure above we have a point source A, placed in front of a plane mirror. Point source A forms an image A’, which can be seen by an observer, as the reflected ray reaches his eyes.
By constructing the figure below, it is stated that triangle AIB is congruent to triangle A’IB, therefore, segments AB and A’B are also congruent. This means that the point object A it's the dot image A’ they are symmetrical with respect to the plane mirror.
Thus, to geometrically obtain the image of a point object, it is enough to draw a straight line through it, perpendicular to the mirror, and symmetrically mark the image point.
By Domitiano Marques
Graduated in Physics
Source: Brazil School - https://brasilescola.uol.com.br/fisica/imagem-um-objeto-pontual.htm