Mirrors Reverse Front-to-Back, Not Left-Right
Contrary to popular belief, a planar mirror does not actually reverse images along a left-right axis. Instead, it reverses the image along the axis perpendicular to its surface, effectively swapping 'front' and 'back'. The perceived left-right reversal is a trick of human perception, stemming from how our brain interprets this three-dimensional reversal when comparing it to our own body.
The Principle of Perpendicular Reversal
When you look into a mirror, your reflection appears as if you were standing behind a transparent pane, looking back at yourself. If your nose points towards the mirror, its reflection's nose points outwards from the mirror. This means objects closer to the mirror appear further in the reflection, and vice-versa. This is a reversal along the depth (or Z-axis), not directly horizontal.
The Role of Human Perception
Our brains are wired to understand the world in terms of our own body orientation. When we see our reflection wave its 'left' hand, it's actually our 'right' hand, but flipped along that front-to-back axis. We mentally attempt to rotate ourselves to match the image, which leads to the erroneous conclusion of a horizontal flip. We don't typically try to do a headstand to match the reflection, thus we don't perceive an up-down reversal.
Why Vertical Orientation Remains Consistent
A simple flat mirror has a vertical surface, and the primary axis of reversal (front-to-back) is horizontal. Your head remains at the top and your feet at the bottom in the reflection because there's no physical mechanism for the mirror to swap these vertical positions. Additionally, Earth's gravity provides a strong, consistent vertical reference point that both you and your reflection adhere to, preventing any up-down confusion.