I know this question sounds trivial, but it has made me so inquisitive over time that I just have to share it here.
When a disc is inserted in an optical drive, it first starts spinning (earlier optical drives tend to check reflection first; no reflection = “no disc”), then I presume the drive tries out all disc types with its laser until it finds the correct disc type and then fine-tunes some laser parameters or similar.
But something I have noticed in all optical drives (half-height and slim type) and even some CD players is the beep after recognizing the disc, probably from the optical pickup unit. For dual-layer discs, there are two of these beeps, which means one per layer. Imminently after the beep, the disc accelerates and the computer can access the data.
I know it is a part of the disc recognition process, but what exactly is it technically?