Did you know

1. The capacity of a DVD-ROM can be visually determined by noting the number of data sides, and looking at the data side(s) of the disc. Double-layered sides are sometimes gold-colored, while single-layered sides are silver-colored, like a CD. One additional way to tell if a DVD contains one or two layers is to look at the center ring on the underside of the disc. If there are two barcodes, it is a dual layer disc. If there is one barcode, there is only one layer.

2. A CD is read by focusing a 780 nm wavelength semiconductor laser through the bottom of the polycarbonate layer. The difference in height between pits and lands is a sixth of the wavelength of the laser light, leading to a third-wavelength phase difference between the light reflected from a pit and from its surrounding land. The sixth, 125 nm, (and not a quarter) of the wavelength was chosen to have a good trade-off between the push-pull radial tracking signal and the full-aperture read-out signal. The interference reduces the intensity of the reflected light compared to when the laser is reflected by just a land. By measuring this intensity with a photodiode, one is able to read the data from the disc. The pits and lands themselves do not represent the zeroes and ones of binary data. Instead a change from pit to land or land to pit indicates a one, while no change indicates a zero. This in turn is decoded by reversing the Eight-to-Fourteen Modulation used in mastering the disc, finally revealing the raw data stored on the disc.