Mirrors flip objects horizontally and not vertically, but why is it?
Take, for example, when facing a mirror and holding up signage. The reflection of the signage in the mirror appears to be flipped horizontally, wherein the word 'Food' becomes 'dooF,' but its letters are still the right way up.
Another example is when raising the right hand in which the mirror would reflect a left hand raises up, but it still moves up rather than down. So, how do mirrors know which is the X direction from the Y direction?
The Z-Direction
In Physic's Girl 2015 video, she explains how reflections in mirrors are horizontal and not vertical. She said that things always appear horizontal because they are flipped horizontally.
But if the 'Food' sign is flipped vertically to face the mirror, it still spells 'Food,' but the letters are vertically flipped. She goes on to explain another example using arrows.
When the arrow is pointing to the right, so does its reflection; when it points up, its reflection also points up. The same thing happens when it is pointed to the left direction and down. But when it is pointed forward, the reflection goes backward, which means the mirror is flipping the image in the Z direction, causing confusion in the direction of reflections.
The Z-direction is the axis in the three-dimension Cartesian coordinates usually oriented vertically. Its concept seems to sound confusing since it is challenging for most people to imagine the direction when most Cartesian planes only talk about the X and Y axis.
Moreover, people rarely notice that objects appear horizontally flipped when looking at mirrors because people are horizontally symmetrical.
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Why Do People Think Reflections Are Flipped Horizontally When They Are Not?
In people's everyday lives and in using mirrors, no one usually thinks that reflections are horizontally flipped, although that is precisely what they do when they put their t-shirts on and see its reflection in the mirror, wherein the shirt is flipped away from the usual reading position, Physics Girl said on her video.
Additionally, most people are left and right symmetric, which causes them to see themselves in the mirror as a normal-looking person.
As mentioned earlier, people are horizontally symmetric in a way that to see their reflection at a normal angle, the mirror will show a reflection that has the same direction as the object. But when the person is standing, and the reflection shows a flipped image of hands on the ground and feet on the air, then that is a vertical reflection.
A vertical reflection of a person looks very wrong just by looking at it because people are not vertically symmetric.
So, to answer the question as to why do mirrors flip things horizontally and not vertically, Physics Girl said that it is because objects are flipped horizontally. The mirror merely shows how the person or the object is presented in front of them.
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