Reflection

Look at the front of an ambulance in your mirror and the word across its bonnet suddenly makes sense. It was painted back to front on purpose, because the only driver who needs to read it is the one looking at it through a piece of silvered glass. Somebody in a paint shop had to know, in advance and exactly, what a mirror does to a shape. They were not guessing. Reflection is one of the most precisely predictable things in the whole of physics, and the rule behind it fits into nine words.

This lesson turns that rule into a skill. You will learn where the normal is drawn and why every angle in this topic is measured from it rather than from the shiny surface itself, which is where most of the marks on this topic are quietly thrown away. You will then learn to build a ray diagram: to place the image behind the mirror by construction, find the exact point where the light bounces on its way to somebody's eye, and follow how that path shifts when the object moves, when the eye moves, or when the mirror itself is turned. By the end you will be able to say why the letters on the ambulance are reversed left to right and not top to bottom, which is a question far more people get wrong than get right.