Why do mirrors flip left and right but not up and down? @Mathologer

Uploaded May 2016 | Updated September 2026, 2 weeks ago

This is a question that people have been puzzling over ever since we discovered mirrors. A really simple comprehensive answer is surprisingly hard to pin down. Find out whether the Mathologer's answer ticks all the boxes.

English subtitles contributes by Anthony Whittington. Thank you very much Anthony!

Enjoy :)

Why do mirrors flip left and right but not up and down?

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$Ptolemy’s Theorem and the Almagest: we just found the best visual proof in 2000 years We are making history again by presenting a new visual proof of the 2000+ years old Ptolemys theorem and Ptolemys inequality. 00:00 Introduction 04:27 Geometry 101 08:19 Applications 14:46 Ptolemys inequality 18:34 LIES 25:35 Animated proofs 28:57 Thank you! 30:53 Degenerate Easter Egg There are some other proofs of Ptolemys theorem/inequality based on scaling and aligning suitable triangles. However, none of them is as slick, beautiful and powerful as Rainers new proof. In particular, check out the animated scaling proof on the wiki page for Ptolemys theorem (and this https://youtu.be/ZK08Z5A9xH4) and check out the scaling proof by Claudi Asina and Roger Nelson: Proof Without Words: Ptolemy’s Inequality in Mathematics Magazine 87, (2014), p. 291. https://www.jstor.org/stable/10.4169/math.mag.87.4.291 Rainer was inspired by a classic scaling based proof of Pythagoras theorem that I presented here https://youtu.be/p-0SOWbzUYI?si=GeGzZ0R_Dj1AsXqR&t=371 You can find a couple of full text versions of the Almagest here https://www.wilbourhall.org/index.html#ptolemy https://classicalliberalarts.com/resources/PTOLEMY_ALMAGEST_ENGLISH.pdf For more background info check out the very comprehensive wiki pages on: Ptolemy’s theorem https://en.wikipedia.org/wiki/Ptolemy%27s_theorem Ptolemy’s inequality https://en.wikipedia.org/wiki/Ptolemy%27s_inequality Claudius Ptolemy https://en.wikipedia.org/wiki/Ptolemy The Almagest https://sco.wikipedia.org/wiki/Almagest Trigonometric identities https://en.wikipedia.org/wiki/List_of_trigonometric_identities Cyclic quadrilateral https://en.wikipedia.org/wiki/Cyclic_quadrilateral The optic equation https://en.wikipedia.org/wiki/Optic_equation There are very interesting higher-dimensional versions of Ptolemys theorem just like there are higher-dimensional versions of Pythagoras theorem. I did not get around to talking them today. Google ... Highly recommended: T. Brendan, How Ptolemy constructed trigonometry tables, The Mathematics Teacher 58 (1965), pp. 141-149 https://www.jstor.org/stable/27967990 Tom M. Apostol, Ptolemys Inequality and the Chordal Metric, Mathematics Magazine 40 (1967), pp. 233-235 https://www.jstor.org/stable/2688275 https://demonstrations.wolfram.com/PtolemysTableOfChords/ an interactive exploration of Ptolemys table of chords Ptolemys theorem made a guest appearance in the the previous Mathologer video on the golden ratio: https://youtu.be/cCXRUHUgvLI Here is a nice trick to make Ptolemy counterparts of Pythagorean triples. Take any two sets of Pythagorean triples: 5² = 3² + 4², 13² = 12² + 5², and combine them like this: 65² = 13² × 5²= 13²(4² + 3²) = 52² + 39²= 5²(12² + 5²) = 60² + 25². Now combining the two right angled triangles 52-39-65 and 25-60-65 along the common diagonal in any of four different ways gives a convex quadrilateral with all sides integers. Note that you automatically get 5 integer lengths and then Ptolemys theorem guarantees that the remaining side is a fraction. Scaling up everything by the denominator of that fraction gives one of the special integer-everywhere quadrilaterals. See also Brahmagupta quadrilaterals. Here is a nice application of Ptolemys theorem to a International Maths Olympiad problem https://www.youtube.com/watch?v=NHjtHOE1lks In a cyclic quadrilateral the ratio of the diagonals equals the ratio of the sums of products of the sides that share the diagonals end points: https://www.geogebra.org/m/XQr5jJQg This extension of Ptolemys theorem is part of the thumbnail for this video. T-shirt: cowsine :) Music: Floating branch by Muted and I promise by Ian Post. Enjoy, burkard$