Compact disks are made of 1.2 mm thick polycarbonate plastic with a refractive index of 1.55. The laser light is focused down to 1 µm, but at the surface the diameter is 0.8 mm, making the readout insensitive to dust specks.
<a href="//commons.wikimedia.org/wiki/User:Pieter_Kuiper" title="User:Pieter Kuiper">Pieter Kuiper</a>
<span class="int-own-work" lang="en">Own work</span>