Explain two primary differences between Compact Disc (CD) media and Digital Versatile Disc (DVD) media in terms of storage capacity and the optical technology used.

### Why this is right A standard single-layer Digital Versatile Disc (DVD) stores about 4.7 gigabytes (GB), compared with about 700 megabytes (MB) on a Compact Disc (CD). The DVD’s shorter-wavelength laser can read smaller marks packed more closely together, so more data fits on the disc. ### The concept A CD or DVD is an **optical disc**: a laser reads data encoded as tiny marks along a spiral track. The laser’s **wavelength**, the distance between repeating parts of its light wave, affects how finely it can distinguish those marks. CDs use infrared light with a wavelength of about 780 nanometers (nm), while DVDs use red light at about 650 nm. The shorter wavelength lets a DVD use smaller pits and narrower spacing between tracks, increasing its **data density**, or the amount of information stored in a given area. | Feature | CD | DVD | |---|---|---| | Standard single-layer capacity | About 700 MB | About 4.7 GB | | Laser | Infrared, about 780 nm | Red, about 650 nm | | Data arrangement | Larger pits and wider track spacing | Smaller pits and narrower track spacing | ### What a good answer covers - A standard single-layer CD holds about 700 MB; a standard single-layer DVD holds about 4.7 GB. - A CD uses an infrared laser at about 780 nm; a DVD uses a red laser at about 650 nm. - The DVD’s shorter wavelength supports smaller pits and closer tracks, which raises storage density. ### Terms - **CD**: Compact Disc. An optical disc read by a laser. - **DVD**: Digital Versatile Disc. An optical disc that stores more data than a standard CD. - **MB**: Megabyte. A unit of digital storage. - **GB**: Gigabyte. A unit of digital storage larger than a megabyte. - **nm**: Nanometer. A unit of length equal to one billionth of a meter. - **Pit**: A tiny mark encoded on a disc and read by changes in reflected laser light. - **Track pitch**: The spacing between neighboring turns of a disc’s spiral track. ### Remember A DVD holds more than a CD partly because its shorter-wavelength red laser can read smaller, more closely spaced data marks.