Optical discs: reading with reflected light

Hold a CD up to the light and you see a rainbow. That's light bouncing off billions of microscopic bumps arranged along one track.

The drive reads them like Morse code made of reflections: bright, dim, bright, bright, dim…

• Data is stored along a single spiral track that runs from the centre outwards. (Not rings and sectors like a hard disk.)
• The track is a sequence of pits (dips) and lands (the flat surface between pits), under a thin reflective layer.
• Lands reflect the laser light strongly back. Pits reflect much less back towards the sensor.

"Describe how data is read from an optical disc." Usually 3 to 5 marks.

The marks are for: spinning disc, laser, pits and lands, the reflected light detected, and turning that into binary.

1. A motor spins the disc.
2. A low-power laser beam is focused by a lens onto the spiral track.
3. Lands reflect the light strongly; pits reflect much less.
4. A photodiode (light sensor) detects the reflected light and turns the changes into an electrical signal.
5. A change between pit and land is read as a 1; no change is read as 0s. This gives the binary data.
6. The laser assembly moves steadily outwards to follow the spiral track as the disc turns.

Step 4 is the forgotten one. "The laser reads the pits" misses the sensor that actually sees the light.