Source
More at the link above.
This one’s a bit crazy, but stick with me. Blu-ray discs, like CDs and DVDs before them, consist of a thin layer — or layers — of recording medium sandwiched between two bits of plastic. Data is stored on this medium in a series of pits — small indentations — that are about 75nm long. To read the data, a laser is bounced off the recording medium — where the medium is smooth and untouched (usually referred to as islands), the laser light bounces straight back into a sensor; where the pits are, the laser is reflected differently. Thus, binary data can be stored and read.
In the case of Blu-ray, the binary data isn’t just burnt directly to the disc — compression is applied, and error control codes are added so that data can be recovered in the case of light scratches. Because the error control codes are applied every few bytes, the end result is a disc covered in quasi-random pits and islands that have a recurring pattern every 150 to 525 nanometers. (The iridescence — rainbow effect — of optical discs is caused by this repeating pattern, in case you wondered.)
More at the link above.
