What's blue about Blu-ray DVDs?

Optical discs use light to read and write data, and the color of the light is important. Older disc storage systems used red or infrared light, while the newer Blu-ray uses bluer light - hence the name. (Actually the name is a bit off, since the light is closer to violet.)

To understand why the switch from red to violet is a big deal, keep in mind that light can be thought of as having waves. Red light has a relatively long wavelength of around 700 nm (1 nm is a billionth of a meter) with violet light about half that wavelength.

Conventional DVDs and CDs use red and near infrared light with wavelengths of 650 nm and 780 nm respectively, while Blu-ray systems use violet light of 405 nm.

Waves of a shorter length can "see" smaller structures - so more data can be on the disc they are reading or writing. Hence, the popularity of Blu-ray as the next generation of optical disc format. Why didn't we use shorter wavelengths from the start?

While we have had good, cheap red and infrared lasers for a long time, violet has turned out to be a bit trickier to develop, requiring additional efforts in data encoding. Thus, the delay in its arrival in our techno-lexicon.

Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.