Why Do Mirrors Flip Left & Right (but not up & down)?

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This video is about why words flip left & right (aka horizontally) in a mirror but not up & down (aka vertically). The answer has to do with specular reflection, mirrors being like windows into another world (alternate universes, just with in and out flipped!), and transparency of the things we write on.

REFERENCES
Specular Reflection
https://en.wikipedia.org/wiki/Specular_reflection

PhysicsGirl video about mirrors
https://www.youtube.com/watch?v=vBpxhfBlVLU

Fermats principle

http://electron6.phys.utk.edu/optics421/modules/m1/Fermat's%20principle.htm

Thomas-Fermi screening
https://en.wikipedia.org/wiki/Thomas%E2%80%93Fermi_screening

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Created by Henry Reich