All types of optical discs (CDs, DVDs, Blu-Ray® Discs, etc.) have basically the same components. The outer layer of clear polycarbonate plastic protects the inner layers that contain the data from damage. A layer of a highly reflective metal, usually aluminum, is used to reflect the laser that is used to read the data. The data itself is stored on another polycarbonate layer that contains teeny-tiny little pits. The pits are arranged in spiral tracks, just like a vinyl record (if you’ve ever seen one of those), and are about 100 nm deep and about 500 nm wide. The laser can detect the change in height by measuring changes in how the light is reflected.