*Optical illusions are almost universally intriguing. Young children can completely get them, but they can fascinate adults too. There is something captivating about being tricked by your eyes. And I think they can provide a great opportunity for motivating some geometry.*

Topics in mathematics that involve accurate measurement can sometimes feel a bit* un*mathematical - more science than mathematics. For example, why is 'scale drawing' a topic in mathematics? Is this just a hangover from the days when 'technical drawing' was a marketable skill that was taught in schools? Converting scales is a useful application of ratio, but what is the mathematical purpose of making accurate drawings? Loci and construction are important topics for understanding concepts in geometry, and the central idea that compass constructions are 'exact in principle' seems to me to be important. But should it matter whether students can execute a perfect circle with their compasses or draw an angle of $35^\circ \pm 1^\circ$ using a protractor? Arguably, making neat constructions may depend more on the quality of the student's instruments (such as how well-tightened the screw on their compasses is) and on basic dexterity than on any mathematically-relevant skill. The beauty of mathematics is the ability to carry out exact calculations that mean that a correct mathematical sketch *not* drawn to scale is generally as useful as, or more useful than, an accurate scale drawing. For example, in an astronomical scenario (e.g., calculating the distance to the sun) we can sketch a 1 by 20,000 right-angled triangle, and this is much more helpful than trying to draw this to scale! In mathematics, we develop ways to calculate so that we don't *need* to make accurate drawings, so perhaps the main purpose of scale drawing is to show students how slow and tiresome things would be without mathematics (a Dan Meyer 'headache-aspirin' situation, see Meyer, 2015)?

Nevertheless, there are times when we need students to measure lengths and angles, and it is great when we can find purposeful ways to practise these skills (see Andrews, 2002). I think finding scenarios where there is a real (i.e., uncontrived) *need* to measure can be quite difficult, but optical illusions can be really helpful for this - and are fun in their own right.

There is a good list of many kinds of optical illusions at https://en.wikipedia.org/wiki/List_of_optical_illusions, and this includes things that are extremely weird, such as * Ames room* (https://en.wikipedia.org/wiki/Ames_room). These might be fun to look at and talk about. However, I focus below on some examples of optical illusions that could have obvious, immediate relevance in motivating some primary or secondary school geometry and measurement.

The * Ebbinghaus illusion* (https://en.wikipedia.org/wiki/Ebbinghaus_illusion) is a nice one. The orange discs below are equal in size, but don't look it.

This is just crying out for some measurement with a ruler. Can the diameters *really* be equal? But the centres of the circles are not marked, so how could you be sure you were accurately measuring the *diameter*, and not some other chord?

The * Delboeuf illusion* (https://en.wikipedia.org/wiki/Delboeuf_illusion) is similar. The black discs are in fact equal, but the right-hand one looks larger:

The * Moon illusion* is a nice variation on this (https://en.wikipedia.org/wiki/Moon_illusion).

Without being prompted to do so, when presented with these illusions, students reach for their rulers. And so, of course, if you want every student to do the measuring, then you need to provide the images on paper, as displaying them on the screen allows only one student to do it on behalf of everyone else.

Students could also attempt to create *their own* drawings, some of which are illusory (something looks bigger but isn't) and some not (something looks bigger and *is*), and see if other students can decide which are which by eye - followed by measuring to check.

The * Müller-Lyer illusion* (https://en.wikipedia.org/wiki/M%C3%BCller-Lyer_illusion) provides motivation for measuring the lengths of line segments. The two horizontal portions below are equal in length, but don't look it!

Similar opportunities are provided by the **Ponzo illusion** (https://en.wikipedia.org/wiki/Ponzo_illusion),

Tony Philips, National Aeronautics and Space Adm., Public domain, via Wikimedia Commons |

*(https://en.wikipedia.org/wiki/Sander_illusion), where the two purple diagonals below are actually*

**Sander illusion***equal*in length,

*(https://en.wikipedia.org/wiki/Vertical%E2%80%93horizontal_illusion),*

**vertical-horizontal illusion**Asking students to attempt to draw a square by eye, using a straight edge (i.e., not a scaled ruler), can be revealing. Then the students measure everyone's and do some statistics to see whether, among the various drawings produced, 'squares' that are tall/narrow are more prevalent than ones which are short/wide. (The easiest way of keeping track of the orientation of each piece of paper is to have the students write their name at the top.) There are similar opportunities for statistical analysis in devising a way to decide how to judge the quality of people's freehand circles (see Foster, 2015, and Bryant & Sangwin, 2011).

The * café wall illusion* (https://en.wikipedia.org/wiki/Caf%C3%A9_wall_illusion) is a bit more sophisticated, and this can be a good opportunity to encourage students to use precise language. What exactly do they mean by 'wavy', 'wonky' or 'not straight'? Do they mean

*sloping straight*lines or

*curves*? "Say what you see" can be a really a useful prompt to use with these figures, and you can follow up with requests for greater clarity.

Fibonacci, CC BY-SA 3.0 http://creativecommons.org/licenses/by-sa/3.0/, via Wikimedia Commons |

The

*(https://en.wikipedia.org/wiki/Z%C3%B6llner_illusion) provides another opportunity for students to check parallelness,*

**Zöllner illusion**Fibonacci, CC BY-SA 3.0 http://creativecommons.org/licenses/by-sa/3.0/, via Wikimedia Commons |

and the * Hering illusion* (https://en.wikipedia.org/wiki/Hering_illusion) is another example:

Fibonacci, CC BY-SA 3.0 http://creativecommons.org/licenses/by-sa/3.0/, via Wikimedia Commons |

For working with polygons, the * Ehrenstein illusion* (https://en.wikipedia.org/wiki/Ehrenstein_illusion) is useful. What do we need to check to see if the shape really is a square? Is it enough just to measure the lengths of the four sides? Is it enough just to check that the angles are all right angles (and how many do we need to measure to do this?)? (The

*- https://en.wikipedia.org/wiki/Orbison_illusion - provides similar opportunities.)*

**Orbison illusion**Often, students address measurement objectives by spending lesson time measuring arbitrary line lengths or angles on a sheet, merely to improve their skill at measurement. Optical illusions can provide a rich context for doing similar work, where there is a motivation to discover whether, say, two lengths really are the same or not. I find that students will measure much more accurately, and with considerably more enthusiasm, when it has some purpose behind it, and I would call tasks like these * mathematical etudes* (http://www.mathematicaletudes.com/) for measurement.

### Questions to reflect on

1. Do you find these optical illusions engaging? Would your students?

2. How could you use these ideas to promote a *need* for measurement with one of your classes?

3. What other tasks make measurement a meaningful mathematical activity?

### References

Andrews, P. (2002). Angle measurement: An opportunity for equity. *Mathematics in School, 31*(5), 16–18. https://nrich.maths.org/content/id/2855/AngleMeasurement.pdf

Bryant, J., & Sangwin, C. (2011). *How round is your circle? *Princeton University Press.

Foster, C. (2015). Exploiting unexpected situations in the mathematics classroom. *International Journal of Science and Mathematics Education, 13*(5), 1065–1088. https://doi.org/10.1007/s10763-014-9515-3

Meyer, D. (2015, June 17). If math is the aspirin, then how do you create the headache? [Blog post]. https://blog.mrmeyer.com/2015/if-math-is-the-aspirin-then-how-do-you-create-the-headache/

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