What experience confirms that the distances from the image to the mirror and from the object to the mirror are equal?

Consider an experiment that allows using a ruler to determine the distance between a point light source and its image in a flat transparent glass (mirror). Place a lighted candle (or light bulb) in front of the vertically mounted glass. On the other side of the glass, put the same, but unlit candle. Moving the second candle, we will achieve the alignment of the image in the glass of a lit candle with an unlit one. Having measured the distance from an unlit candle to the glass, make sure that it is equal to the distance from a lit candle to the glass.