The phenomenon you're referring to is known as "stroboscopic effect" or "wagon-wheel effect." It occurs when the motion of an object appears to be slower or in a lower frame rate when observed through a rapidly oscillating or rotating object, such as a spinning wheel or a moving fan.
The stroboscopic effect arises due to the way our eyes perceive and process visual information. Our eyes and brain work together to create the perception of motion by integrating a series of still images, or frames, into a smooth and continuous movement. However, when the frequency of the oscillating or rotating object is close to the frame rate at which our eyes perceive the motion, it can lead to an apparent slowing down or freezing of the motion.
To understand this effect, imagine a wheel with spokes rotating at a certain speed. As the spokes move, they block and allow light to pass through at regular intervals. If the rotation speed of the wheel matches the frame rate at which our eyes capture the images, it can result in an illusion where the object appears to be moving slowly or even in the opposite direction.
The stroboscopic effect is not limited to mirrors; it can occur with any oscillating or rotating object and is a result of the interaction between the object's frequency and the frame rate of our visual perception.