How does a CD player “read” a CD and play music? What is physically happening here?
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So juts like your hard drive, a disk stores binary data. But instead of a magnetic disk, it’s engraved. On some disks, especially blue rays, you can kind of see how much of the disk was written to. But the player has a laser that bounces off the disk into a reader. If it gets to a groove, it will take slightly longer to return, and the reader can pickup on that and translate the bump into digital data that the computer inside can read
The CD has multiple layers of plastic, some clear and some reflective. A laser is shot onto the surface of the CD and based on thickness of the clear parts you get a 1 or a 0. One area might be 1/3 thickness clear an 2/3 reflective and others are 2/3 clear and 1/3 reflective. Based on the angle of the laser and the thickness the reflection moves around and a sensor can tell how thick it is based on that reflection.
A CD holds data the same way as a computer: a series of 1s and 0s that are organized so that the computer (including the smaller one on a CD player) recognizes it as a series of sounds.
To physically read the 1s and 0s, the reader shines a laser at the CD and can catch the laser’s reflection with a sensor. However, some parts of the CD are changed so that the laser doesn’t reflect. These spots are arranged so that the reader can go over all of the CD and consider the parts it saw a reflection as a 1 and no reflection as a 0. The reader sends the order of 1s and 0s to it’s computer, which turns them into data and then music.
As OP’s question has been answered I will just add this is why when CD-R was brought out, it was called ‘burning’ a disc. The ‘write’ laser in the CD-R drive literally burned grooves (or left it alone) into the material on the CD-R disc.
Adding to the other answers, if you want to know more there’s a series of videos made by Technology Connections. [Here’s the playlist](https://www.youtube.com/playlist?list=PLv0jwu7G_DFWBEyCKt4tKHIk8ez_pZS_P), you can watch only the introduction (The Compact Disc: An Introduction) for the general idea, the other videos try to focus on specific things.
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