Let's take apart a CD/DVD drive, layer by layer

A motor pulls the tray in and out

Move to the side to see how the rotation motor is lifted

A rail and gear push a cam that lifts the motor, driving its spindle through the disk

And then there is the mechanism that moves the optical read/write head . . .

The optical head is the heart of the CD/DVD drive.

Here are its basic components.

The laser passes through a splitter and lens, bouncing off the disk to the detector.

This is the path followed by photons of beam.

The disk's pattern of pits is translated into an optical signal.

But disks are not flat . . . so things get complicated.

So the lens is suspended inside an electromagnet.

But we need to figure out how the lens should move.

Simplfy things by straightening out the beam's path.

Look more closely at the point where the laser focuses.

Trace light rays as they pass through the lens and its clone.

If the disk moves, the beam no longer emerges parallel.

Fold the path back up and see what happens at the photodetector.

We could use a circle and ring detector.

But most CD/DVDs use a more complicated scheme.