Mirrors reverse left and right because they flip the image horizontally. This means that if you stand in front of a mirror, your left hand will appear to be on your right side, and your right hand will appear to be on your left side. However, mirrors do not flip the image vertically. This means that if you raise your hand, your reflection will also raise its hand.

The reason for this has to do with the way that mirrors work. Mirrors are made of a reflective material, such as silver or aluminum. When light hits a reflective surface, it bounces off of it and travels back in the direction it came from. This is why you can see yourself in a mirror.

When light hits a mirror at an angle, it is reflected at the same angle. This means that if you look at yourself in a mirror, your reflection will be the same size as you and it will be located directly opposite you. However, if you move to the left or right, your reflection will move in the opposite direction. This is because the light from your body is bouncing off of the mirror and traveling back in the direction it came from.

The same thing happens when you raise or lower your hand. The light from your hand is bouncing off of the mirror and traveling back in the direction it came from. This means that your reflection will raise or lower its hand at the same time that you do.

This is why mirrors reverse left and right but not up down.