PHYSICS WORTH SHARING
Hold a sticky note with a big R up to your forehead. Look in the mirror. The R looks backwards.
So mirrors flip left and right… right?
Wrong. They flip front and back.
Big idea
A mirror reverses the axis perpendicular to the glass — depth. Your left hand is still on your left; what's "swapped" is that the image is facing you, so it feels like a left-right flip.
Write TROUBLE on paper and hold it up to a mirror: it looks like ELBUORT. Flip the paper top-to-bottom instead and the mirror shows it upside down, not backwards. The mirror never cared about left vs right. It cared about near vs far.
Try this (2 minutes)
For: Waves & light / optics intro. Materials: clear plastic (overhead transparency or sheet protector) + sticky note + marker + mirror.
Write a big R on clear plastic so it reads normally to you.
Ask: "Does the mirror flip left-right or up-down?"
Hold the plastic between you and the mirror. The reflection reads correctly. Then stick the same R on your forehead facing out and look again — now it reads backwards.
Land it: front↔back reversal. Left stays left. Paper only looks "flipped" when you turn it around to face the glass.
Summary
Mirrors reverse depth (front↔back), not left↔right.
Feeling flipped ≠ the physics of the reflection.
Classroom tell: clear plastic (reads correctly) vs forehead letter (reads backwards).
Shawn's Footnote
If a free-body diagram of light ever needs a "left-right force," you've already left the building. Specular reflection is one plane at a time: the normal flips the component along itself. Everything else keeps its sign. The mystery is anthropology, not optics.
Forward this to a colleague who still says "mirrors reverse left and right."
Stay Wildly Curious,
Lauren & Shawn
