1Two things light does
Light goes straight — until a glass wall bounces it back
You only need two ideas. Watch each one:
Light travels straight
Point a beam and it shoots in a straight line until it bumps into something. On its own it never curves.
Glass can bounce it back
When light inside glass skims the wall at a shallow angle, the wall acts like a mirror and bounces every bit of it back inside.
2Two ways light can hit the wall
A steep hit leaks — a grazing hit bounces
It all comes down to the angle the light hits the inside wall. There's a tipping point between two cases:
Leaks out 💧
Light hits the wall closer to head-on. Much of the light passes into the air; a smaller part reflects inside.
Bounces back ↩️
Light skims along the wall at a shallow angle. The wall reflects all of it back inside, like a mirror.
3Your turn — aim the beam
Tilt a ray inside a straight glass thread
This straight-glass practice uses angles that all give total internal reflection. Change the aim within that range and watch the bounce spacing.
4Now bend the thread
Does one perfect bounce guarantee the next one?
Follow two wall encounters along a glass thread. The first incidence angle is 60° from the normal. At the second encounter, the wall has turned: the angle is 30°. The glass and outside air stay the same.
Guess before you send the light
The first encounter reflects the whole pulse. Does that guarantee the whole pulse stays inside at the second encounter too?