Reflection & Refraction: Light Changes Lanes
Two rules govern every mirror, pool illusion and glass fibre: the bounce is symmetric, and the bend comes from a speed change at the border.
Builds on: 4.1 Waves: Motion Without Transport
The bounce: reflection
Light hitting a surface bounces off at the same angle it came in — angle in = angle out, both measured from the normal (the line perpendicular to the surface — optics’ bookkeeping convention, worth adopting immediately). A mirror is just a surface smooth enough that all the parallel rays bounce in step; a painted wall reflects the same light but scrambles the directions, which is why you see the wall and not yourself.
The bend: refraction
Light’s vacuum speed c is the universe’s limit, but in a medium light is slowed — in water to about c/1.33, in glass c/1.5. The slowdown factor is the index of refraction n. And a speed change at a border does something geometric: a ray crossing the border obliquely changes direction.
The marching-band picture makes it intuitive: a rank of marchers hits a muddy field at an angle; the first marchers into the mud slow down while the rest still stride, and the whole line pivots. Every “broken” drinking straw and too-shallow-looking pool is this pivot: light from the submerged part bends at the surface, and your brain — which assumes straight lines — reconstructs the object in the wrong place.
The one-way mirror in the pool
Going from slow to fast (water toward air), the ray bends away from the normal — so there is an incoming angle whose exit would be 90°. Beyond that critical angle (~49° for water), the light cannot leave at all: it reflects back down, perfectly — total internal reflection. From underwater, the surface beyond that cone is a flawless mirror.
That flawless mirror is a technology: an optical fibre is a glass thread in which light ricochets by total internal reflection for a hundred kilometres, losing almost nothing. This page reached you through exactly that trick.
A fish seen from the bank is not where it appears — refraction lifts its image. Aim below what you see. Herons know this without the formula; now you know it with one.
⚗️ Lab — The Light Bender
Aim a ray at a border between two materials and read both angles honestly.
- Air into glass: watch the ray duck toward the normal, and check Snell’s numbers.
- Reverse it — glass into air — and raise the angle until the ray refuses to leave.
- That refusal is total internal reflection: the physics your internet rides on.
Work it by hand — 0 / 2
No multiple choice here: compute the value and type it. Suffixes like 2.5k or 20m are understood; answers within ±2% count.