The phenomenon you're referring to is known as mirror reflection or mirror reversal. It's true that when you look at yourself in a mirror, the image appears reversed horizontally but not vertically. This effect occurs due to the way mirrors reflect light.
To understand this, let's consider how a mirror works. Mirrors are typically made of a flat, smooth surface with a reflective coating on the back. When light hits the mirror, it undergoes two reflections: one at the front surface of the mirror and another at the reflective coating on the back. The light then travels back toward your eyes, allowing you to see the reflected image.
During these reflections, the mirror reverses the direction of the light rays in the horizontal plane, while the vertical direction remains the same. This reversal happens because the reflective coating on the back of the mirror causes the light to bounce back in the opposite direction, flipping the image horizontally. However, the vertical position of the light rays remains the same, so the image is not reversed vertically.
To visualize this, imagine you're standing in front of a mirror holding a book. The top of the book corresponds to the top of the image you see in the mirror, and the bottom of the book corresponds to the bottom of the image. However, the left side of the book appears as the right side in the mirror, and vice versa. This horizontal reversal gives the mirror image its characteristic flipped appearance.
It's worth noting that the perception of the mirror-reversed image is something our brains have learned to interpret. We are accustomed to seeing ourselves and objects in mirrors from an early age, so we adapt and recognize the reversed image as our own reflection.