I am currently working as a teacher under the illustrious and all-powerful Richard Buckland, well known for encouraging critical, scientific thinking among his students and among his tutors.

I was very amused when I read a certain question in the tutorial questions today (or rather, yesterday). While not exactly being relevant to computer science per-se, it certainly is relevant to scientific thinking generally. It forced my students to think with a precision of logic (and skepticism) about the world around them that I do not usually see:

"Students: explain to your class why mirrors reflect left and right but not up and down."

I thought this question was probably one of the best questions I've seen all year. Before you go on reading this post, you too should stop for a minute and think about it.

(slight spoilers follow for those who did not catch the trick in the question)

Every single one of my students immediately thought it was some side effect of the brain's interpretation of the mirror's image. So, socratically, I answered their explanation with a question.

If you turn your head ninety degrees, so that you still look at it orthogonally, but your eyes are oriented differently - why does the image not change on the mirror? If your brain is correcting based on your orientation, wouldn't your brain correct it so that instead the image flips differently in that case?

This sent them back thinking for a while, and eventually they concluded that the mirror must be somehow engineered so that the image is only inverted on one axis, although some students were a bit skeptical of this idea.

I immediately came up with a counter-question much like the first.

Then, if you rotated the mirror ninety degrees, would the image not flip vertically?

Eventually, one of the students used a blanked out laptop screen as a mirror, wrote the word "Hello" on a piece of paper, and tried to do some experimentation. Of course, experimentation is scientific, so this drew my approving applause.

I will post his monologue here:

So, I've got a word on this piece of paper, it's the right way around now. If I flip this piece of paper so that it's facing the mirror... wait.. wait a minute..

I began applauding again, and eventually the students came to the correct conclusion - of course, my entire question is based on a false premise. Mirrors do not reflect left or right, so they most certainly do not reflect up or down. In the traditional misleading case where text in a mirror image appears backwards - this is simply a matter of perspective. It is you who has flipped the piece of paper causing the text to be inverted, not the mirror(*).

I think this question is a superb example of skepticism and semantics. The assertion that mirrors reflect left and right seems right, but it is entirely a misleading assertion. The fact that it is presented as an assumption, laying the groundwork for the main question, reinforces the belief that the assumption is true, when reality is quite the opposite.

I believe this question teaches a valuable lesson about critical analysis, skepticism of presented facts, and helps to promote understanding by experimentation. All of these are essential qualities in any science academic.

(*) - If you had an identical twin who wrote a message on a piece of paper, and held it up so that you, who is facing your twin, can read it, you may notice that neither party would have any difficulty doing so. This is because in this case, the identical twin has been flipped also, whereas in the mirror example the reader and the writer are the same person, so their orientation has not been inverted. The mirror example is really more like having an identical twin that reads from right-to-left.

I'm going to be teaching this again today, I'll update if I have more interesting stories.