mouse - How does a Logitech scroll wheel detect movement?

I just replaced my scroll wheel in my mouse, a one minute repair. But now I've seen something that can't be unseen.

Logitech scroll wheels apparently function using magic.

It freely rests on three buttons that detect left, right, and straight down presses. But the wheel itself is not electrically connected to anything whatsoever. It's plastic resting on a spring.

How does the mouse know when I'm scrolling? And why was it affected by dust and gunk?

Edit: and how does it detect which direction you are scrolling?

Answer

There's a diode (probably infrared) and a receptor, one on either sides of the wheel, soldered directly into the green PCB. Wheel is SPOKED, and by turning it you interrupt light travelling from the diode to the sensor. No magic there.

Oh, and dust sitting in there can permanently block sensor from any light coming from the diode, so there's no way for it to register interruptions, that's why the wheel appear to not work.