So how do optical discs work?

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How on earth does data get written onto that material? And how the hell can lasers read that data?

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A pre-made CD/DVD or whatever has layers of metal inside it that have microscopic holes cut in them. When the laser is fired at the disc the reflection is different depending on whether it’s hitting metal or hole, and that difference can be detected and used to read the 1s and 0s of binary data. Writable media uses a dye layer but operate in fundamentally the same way–the dye is set up so you can change its reflectivity using a stronger laser blast than is used for reading it, and the differences in reflectivity can be read by the laser in the same way. (It’s not *quite* the same, which is why some old CD drives weren’t able to read writable media properly).