Photolithography is a key step in integrated circuit fabrication, where light is used to define and etch a pattern onto the silicon wafer.
Defining areas of interest on the wafers is done with photoresists. They are used to coat the wafer, then illuminated through a photomask.
The photomask is constructed with chromium covered quartz glass. The chrome determines where light should be blocked, and a UV light is shone through to expose the photoresist where there are gaps in the chrome.